Looking into debuff inconsistency today. This stuff is a hot mess.
That is all, I think there's a lot of good to be done here.
This is the spreadsheet I created of enemy immunities, along with my suggestions. There are separate pages for Hardmode and pre-Hardmode, as well as for each event. My intention with these suggestions is to resolve inconsistencies, render certain debuffs more viable by removing immunities, balance other debuffs by adding immunities to them, and add immunities to enemies which should realistically have them.
(This was created using information from the Wiki, so I can't personally attest that it is thoroughly accurate, but I trust the judgement of the Wiki editors.)
Some major alterations:
β There are no longer any enemies with universal debuff immunity, which fixes the issue with summon tag damage. Some of these enemies now have few immunities, while others have extensive but carefully-selected lists.
β Most enemies (with the exception of Underworld enemies and spirits) have lost immunity to Cursed Inferno and Shadowflame.
β Spirit enemies have gained immunity to Frostburn and Ichor.
β Crimson enemies immune to Poison/Venom now have Ichor immunity instead.
β Many Jungle enemies have lost Poison/Venom immunity (and so have spider enemies).
β Many Ice biome enemies now have immunity to Frostburn (and so do Frost Legion enemies and several Frost Moon enemies).
β Many fighters and flyers are no longer immune to Confused.
β Pirate Invasion enemies have lost Poison/Venom immunity.
β Numerous Solar Eclipse and Martian Madness enemies have lost Confused immunity.
β All Solar Pillar enemies now have immunity to On Fire, with several also losing immunity to Confused.
β Several mechanical enemies have gained immunity to Poison/Venom and On Fire, most notably the Twins.
β The Empress of Light has gained immunity to Poison/Venom and On Fire, reflecting her ethereal nature.
β Duke Fishron and the Lunatic Cultist have lost their immunities to Poison/Venom and On Fire.