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.
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.