I don't really have a ton to say about them because I am bad at video games. I just despise the mechanic of Permadeath+Randomization. Everything about the genre screams "get good" while punishing all the central premises of learning. It's just random difficulty while feeding you semi-similar levels on the supposed "beginner levels". Every so often you'll have some bullshit happen that wipes all of your progress, with not a single save point in sight.
Look I'm playing games to have fun, not to feel like some super badass that has mastered every possible mechanic a game can throw at you based solely on the beginner levels. It just feels like the genre exists to punish you, rather than to foster enjoyment.
Widely accepted? I'd disagree. A heart-killing deck needs a bunch of utility even just to get to that point. In ironclad alone, you want cards that remove strength, give AoE, benefit from exhaust, apply statuses, block, and run your own scaling. There's a reason corruption is a rare card, and why dead branch + corruption is basically a win. Sure, a 15 card deck may be able to out-scale the bosses, but chances are you'll be putting in a lot of cards.
In addition, 40 cards is not that big of an issue. You have many cards that give you draw, and in the late-game, you want draw/tutor to help pull out the right cards at the right time. What's more, some of your cards set-up/exhaust, so your deck will get trimmed down in the first 10 turns or so.