I actually thought about it a bit longer and realized, its not really the bosses being tanky, its just the guns being really underwhelming damage-wise
like for example, I got a railgun on my first run of the game, so I figured "man this would probably deal a ton of damage to the boss, I better save it up for them"
so I bring it to the boss, pull it out, charge it up and blast them with it...
...and it does the same damage as just a single clip of the starter pistol.
the guns look and act unique, sure, but all of them are underwhelming when it comes to damage personally.
Nuclear Throne did a great job of making its weapons as powerful as they feel while being balanced, Enter The Gungeon's guns are just
Awesome, but impractical.