On a solo this an easy one: devices regardless of the map. I get the enemies that love to spawn on top of the objective and refuse to respond to kiting other than the occasionally dumb basic trooper that sticks his head up just enough to get headshotted to make room for another one; when this doesn't happen a convenient grenade or air humper will get me with 1/2 a second left resulting in the whole goddamn process having to start again (if there was some drain while off I would say fair but a complete restart, in my opinion, is cheap/artificial difficulty) If devices come up on wave 10 of hydra or jade and I'm soloing, I don't even bother to try once the aforementioned bs ones come up.
With a team: as has also been stated it depends on the PUGs. I've seen the hardest devices get finished in almost no time with some teams only to the have the next ones somehow almost manage to fail the walk up the hallway escort on hydra when the enemy spawned in back corner they come from on extraction.