There's nothing to really explain, it is exactly how it sounds. Bosses in high-end content tend to have high debuff resistance, and you won't be able to rely upon them for your damage (you'll have to work on your grid instead). Rather than being must-haves, debuffs becomes a nice bonus IF they land (and ofc debuff that guarantees to land become extremely valuable). What typically happens is you drop your pure debuff slaves if you got anything better. Those units that can only debuff or are reliant on them becomes niche, relegated to fighting enemies that aren't debuff resistant.
Typically, the order for those 5 is Gaia > Brahma > Acala = Ares > Ea. It depends on your team and priorities how you want to shift those order around. Although with thunder tower on the horizon, I don't think you should discount Gaia completely.