Gear checks vs. actual fights
Terraria bosses split cleanly into two groups, and good tier lists say which they're ranking. Some are progression gates — beat them by showing up with the right armour and arena, no skill required. Others are genuine bullet-hell duels where Duke Fishron and Empress of Light earn their reputation by punishing every mistake regardless of gear. Difficulty also swings wildly with class and world difficulty: a boss that folds to a melee build can wall a summoner. Rank by design quality if you want a defensible list, by pain if you want an honest one.