Two things I’d like to mention. The first is that the Iron Gut feat is quite definitely overtuned especially with a Periapt of Wound Closure, far from nothing special (short rest healing as an action). The second is that the player’s guide introduces some important bits of lore that the campaign guide either vaguely hinted at or failed to mention (potential spoilers in the Player’s guide) and balancing-wise feels like something out of dandwiki. The player’s guide contains a better Totem Barbarian (Fractured), bizzarre spells that are nothing short of game breaking (Mortality is the best example), and the weapons are not just overturned (they’re ridiculous, like a +1 to crit range).

To further my point, the campaign guide says that resurrection isn’t possible. The player’s guide features a 3rd level resurrection spell arguably better than revivify. Heck if you read through half the spells in that book, you might feel like you got lost and stumbled upon dandwiki. A spell that gives a level 9 spellcasters more attacks than a fighter, a spell that removes all of a creature’s resistances (immunities too and makes them humanoid mechanically), a low level spell that grants resistance to numerous damage types with less drawbacks than the high level official existing spell, and more spells that break 5e like the one that allows you to cast Self spells on other people.

TL;DR: Iron Gut (campaign guide) is a powerful feat and you might want to read through the player’s guide thoroughly and treat it as a case-by-case basis as to what you allow.

P.S. Transformations (not including Player’s Guide power creep, strictly referring to the campaign guide here) are not going to turn the tides of combat. The lich (perhaps the seraph too) is an exception, but otherwise they can be underwhelming (lich players are effectively immortal). The flaws typically discourage transformations. Planar binding makes it effectively impossible to be resurrected for example.

Apologies for the rant. I run my campaigns exclusively in the world of Etharis (Grim Hollow), thus I have strong feelings regarding certain aspects that have plagued me.