Running a quick test on Spider 20:
A full 10 spiderling consume is 30% MAX HP healing = 1,666,550 HP
So Spider 20 should likely have 5,555,165 MAX HP (since the healing is probably rounding up)
Meanwhile a single hit of 5% Poison hurts her for only 27,776 Poison Damage
Thus 5% Poison actually only does 0.5% Poison instead versus this boss so it seems to be a staggering -90% reduction in capability (ie. it only does 1/10th of the expected damage).
This could likely be just for Spider, or even just for this level of Spider, the unfortunate thing is that it is very hard to know and there is no info in game explaining this. I think it would be really nice if we could get a guide or more info on boss resistances, otherwise having a debuff mechanic which just says 5% but rarely ever actually does 5% seems kind of pointless?