Never leave an affliction that lowers dodge, it will save you in many situations.

An affliction that lowers dodge is always bad, no matter what. An affliction that lowers any resistances is bad, it makes them a target for enemies that bleed or blight, but if you can work with it and throw on trinkets that benefit from those debuffs, it could work, and yet it still won't be worth it 9/10 times. An affliction that lowers damage or accuracy isn't terrible for healers or buffers, but debuffers will suffer with less accuracy, and obviously a dps will suffer massively. This will only be good for low-level dungeons, or in parties where you have a high Surprise chance against the enemy. An affliction that lowers speed is typically bad but, if their speed is high enough to deal with it, it can make them slot neatly into certain comps that can work with it, especially if it allows you to drop some speed trinkets in order to deal more damage. An affliction that increases stress is a big no no, this will bite you in the ass always, and it will make that hero a priority target for stress-inflicters. Depends, Flagallent has a very beneficial affliction but all afflictions make them do random things.

Darkest dungeon afflictions and virtues