For the most part I'd heartily recommend Alex's Mobs, since it adds a ton of biome-appropriate animals that fit the Minecraft art style that make the world feel much more alive.
I had it installed when I last played Minecraft over a year ago, but I didn't make it far enough to encounter its many overpowered hostile fantasy creatures and now they've been responsible for most of my deaths in my current world.
My first encounter with one of them was when I was scoping out an ocean on a boat and was suddenly being killed by a giant skeletal swordfish in a matter of seconds. These guys spawn near shipwrecks, have an insanely long aggro distance and can outrun your boat so you're basically screwed the moment you accidentally wander near a shipwreck you can't even see on the bottom of the ocean.
Then I built a Nether Portal and realized I hadn't yet even seen the new Nether biomes. I enjoyed the view for about 30 seconds until I was swarmed by giant mosquitos that attached to my face and sucked half my healthbar away in one go. I managed to kill a couple but they just kept spawning, instantly making a beeline for me the moment they appeared. Very fun and balanced 
After I'd gotten annoyed and just disabled the mosquitos from spawning I was building a bridge across a lava-filled chasm when a gigantic snake skeleton knocked me off and I burned to death. My first warning that they existed was the second one of them leaped up at me from a dozen blocks below. At least they're only limited to lava lakes, I guess.
Later, I was back in the Overworld exploring a giant deepslate cave when all of a sudden I was being attacked by a shrieking head floating up from the caverns below. After I once again got killed in a few hits I looked up what the hell even happened and it turns out it was this Japanese youkai thing that has an attack range of 50 fucking blocks. Just now I was surveying another deep cave from a ledge, saw one of these spawn in way below and it just aggroed instantly and murdered me.
I mostly like these monster designs but why do they have to be this overly aggressive and powerful? They give you no warning and can outrun you and kill you in seconds- it's just completely out of tune with the way Minecraft's hostile mobs are designed. Even the more dangerous mobs like Ghasts make their presence well known far in advance. I guess they're balanced for players kitted out in enchanted diamond armor but they completely kill exploration for me... until I disable them in the config, but I just wish you could turn their aggro range down instead.
The mod also adds several large dangerous animals, like crocodiles and grizzly bears, but you can just avoid those or run away if you too get too close and they feel much closer to how Minecraft usually operates.
Yeah, this is one of the reasons I always balk at players complaining that "The game isn't challenging enough, I could do a better job designing it"
Because they do stuff like this, no thought put into how any of this stuff meshes into the actual game
Just turning what's a fairly chill game, even when you're spelunking, into a constant assault where your only choice is to be kitted out to the 9's or have the response time of a overclocked 13 year old
Take the Nether for example, many biomes are absolutely crawling with Hoglins and Piglins that can and will gang up to kill you if you're not careful. However, when it comes to the Hoglins, their aggro ranges are not unmanageable, they make plenty of noise alerting you to their presence and you can even ward them off with warped fungus. The Piglins will mostly ignore you if you wear at least one piece of golden armor (at least until you open a chest or try to mine gold) and they also avoid soul torches. With the addition of the gold bartering system, the piglins are probably the most interesting mob in the game, simultaneously dangerous and helpful. I like how they feel like intelligent beings that kind of tolerate you showing up on their turf if you behave.
I hadn't played much with the big Nether update and I really enjoy the mobs and mechanics it introduced. By comparison, most of these hostile mobs just fly up to you from a continent away and kill you in three hits
The default mobs are indeed interesting in that regard because there are ways to avoid or cope with all of them.
Many hostile mobs only come out during the night and burn during the day. A big part of Survival is lighting up an area to claim it as your own and protect it from monsters, and use that space to stay safe. Once you can get a bed it lets you avoid the monsters entirely if you want to.
The nether mobs are more dangerous and tolerate you less, but there's still ways to avoid them if you know what you're doing. Piglins let you wear gold to assimilate like you said; the previous Zombie Pigmen only attacked if you attacked first; Ghasts can be heard from a long distance away, their fireballs can be deflected back at them if you don't have ranged weapons, and they have little destructive power; the various skeletons are very dangerous but they only appear in certain biomes or structures meaning you have to confront them after you're prepared.
For the overworld monsters that come out during the day, they are bound to a specific area or to specific conditions. Raiders are only interested in defending their towers. The water jellyfish things are only interested in defending their temples. Wardens are only interested in defending the deep underground areas. They all have their own turf and you can avoid those areas if you aren't ready for them.
The only vanilla monster that doesn't fit this pattern is the water trident guys. My least favourite memories from the game is trying to set up camp next to a beautiful waterfront after just updating to the aquatic update, and then getting 2-hit by really accurate trident guys. They inhabit the whole ocean and are incredibly strong, which is unfair to new and less equipped players. It means they can no longer travel half the world.
But that doesn't compare to mods like Alex's Mobs where you'll be minding your own business and a jaguar will emerge from behind a tree, be faster than you, and bite you to death.
This kind of shit reminds me of the libertarian weirdo who wrote the "Better Than Wolves" mod back in the day, who incidentally had a meltdown in the Forge modding forums once people started asking for a version compatible with something newer than Minecraft 1.5.
Got pretty lucky, never encountered the centipedes underground. I enjoy the mod but I've had history with it just killing my TPS. I don't know if it'll still do it but back in 1.16 it spawned probably 2000 leafcutter ants from a single nest and my world was nigh unplayable. It does sorta expect you to be in full diamond gear at least for the spelunking part, which doesn't always mesh with spelunking for those exact resources.
The crocs are pretty cool though, mixed with some biome mods you'll be walking through a marsh and just sorta hear them make noises and you gotta get out of the water ASAP. It's a shame there's bullshit mobs when there are genuinely good ones too. Also just like finding the monkeys when I'm exploring a jungle.
I actually like the centipedes, they're frankly terrifying and make the depths feel appropriately intimidating. Crucially, they can't fly and you can run away from them. They're also huge and with their glowing eyes they're easy to see from ways off even in the dark so you can keep a wide berth.
I just hate all the hyper-aggressive and fast flying mobs since it feels like there's nothing you can do to escape them.
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Yeah the mod is constantly adding new mobs and it's a bit hard to keep up honestly. Gotta spend a couple hours going through all the mobs and configs to rebalance it before actually playing.
Yeah Alex's mobs is kind of a weird mod for me since I like the idea of it more than playing with it in practice.
Most of the passive mobs are mostly decorative but have one specific drop that has a single niche function and that's it, ie brewing ingredient, single piece of armor that's useless by midgame, etc.
Others are too rare to be found, others are unintentional nuisances (whales can destroy ice blocks near water, which sucks if you happened to be building with them)
I feel like the mob I've actually fully enjoyed the presence of has being emus, of all things
tbh the lava lake one seems like your L. No investigation, no right to complain. The mosquitos suck.
Playing dawncraft right now, and they add alex's mobs plus a ton of hostile mobs.
It's a bit annoying having hostile mobs everywhere, but it's more engaging because they've added soulslike combat too. Idk tho, i shift pretty freely between creative and survival, so i just opt out sometimes.
It's pretty funny to be cutting down a tree and have a vampire fall out.
Been thinking more about this. Can you just use In Control to disable spawning for the mobs that annoy you? Then you can keep the more aesthetic ones
This is why you can only play with kitchensink mod packs with overpowered tech mods to balance things out.
