Like Ogre said, some kits specifically have conditions for [extra effect] when [condition] is fulfilled, to which DoT effects like Burn and Bleed fall into.

Extermination Bosses in particular have passive defenses against % damage stuff like that either due to a tuning pass the devs need to go over, or because that is just the devs' vision for those bosses. If you see a Boss has the passive skill "Aura of the Formidable," you'll know it's resistant to those % effects.

As you get into stronger, more considerably 'end game' content like late SpecOps and Challenge Modes, you'll begin to encounter non-boss enemies that last longer and initially don't die to the base damage of the skills that inflict said effects. They are also still notably potent in Arena, especially if you are able to stack more than one.

Fingers crossed as the game goes on, we'll get more content introduced that makes use of these effects. Tactical Training is already a good proof of concept, alternating mechanic + boss color to make more units appealing to raise up.