A lot of enemies are currently immune to debuffs they shouldn't. Pretty much any enemy that has universal immunity is immune to debuffs they shouldn't, including Ghosts and half the Pumpkin Moon enemies. It means they're immune to taking damage from the Star Cell's effect (they visually stick but don't deal damage), and presumably also effects like Midas and Dryad's Bane that nothing should have immunity to.
I also feel that, for an effect inflicted by so few items and weapons, too many enemies are immune to Confused. Over 70% of enemies are immune to Confused, according to the Wiki. Including weak insects found in the cavern layer that really don't need the immunity. There's a lot of inconsistency in which enemies are immune – most flyers are (Vultures, Hornets, Harpies), but some aren't (Demon Eyes, Parrots, Alien Hornets). All swimming enemies are also immune. Personally, I think enemies should only be immune to Confused if there's a good reason for it – if they're bosses or elite enemies that are meant to be "unstoppable", if there's a valid thematic reason (enemy doesn't have a brain, like Meteor Heads), or the enemy's mechanics dictate it (worms). For an effect as niche and uncommon as it is, over half the game's enemies shouldn't be immune to it; it should be balanced around the assumption that most things are susceptible to it. This is the reason why the Brain of Confusion was underpowered for so long; it had to have mechanics not related to Confused added before it became worthwhile.
There are also quite a few enemies meant to be mechanical or non-biological (including Scarecrows, Elf Copters, Nutcrackers, and Tesla Turrets) that are currently susceptible to Poison/Venom.
Also, it would be really excellent if the Sandstone Slab could be given an accented variant; it only makes sense that both slabs receive variants. After all, the change was only made to these two blocks (I assume). Adding new items is beyond the purview of this thread, but I hope the possibility is at least considered in future.