The Tortle's Natural Armor trait says:
Due to your shell and the shape of your body, you are ill-suited to wearing armor. Your shell provides ample protection, however; it gives you a base AC of 17 (your Dexterity modifier doesn't affect this number). You gain no benefit from wearing armor, but if you are using a shield, you can apply the shield's bonus as normal.
My question is whether or not this means all benefits from armor are ignored, or if it's just the AC benefits. For example, if a Tortle is wearing adamantine armor, its AC is still 17, but is it still able to be critically hit? Same goes for items like Hide of the Feral Guardian or Mizzium Armor; would you be able to use an action from the armor to cast polymorph on yourself/would you take no damage on successful Strength and Constitution saving throws?