It's really the wrong question. Because it's not the individual units that make the factions fun or otherwise, it's the combinations of enemies and the techniques one needs to evolve to deal with them.
The things that most people hate as BS are the parts where the faction breaks part of the game to its advantage.
For example, Collectors do a lot of damage, fair enough. But then to regenerate shields, one has to take cover. At which point a Seeker Swarm will take away your powers and health, and a Scion will magically carpet the area round you with a dozen grenades (since it can't see you and you can't see it, this is mega BS) that will stagger you out of cover so that you can be taken down by that Captain that just spawned five metres behind you.
Another one, Geth Hunters. The problem isn't that they are sneaky, it's that their cloak makes them invincible. You can't melee them and often grenades don't work when they are cloaked. This is BS because the Hunter is still there. (same reason Stasis is wrongly set up but that's a different thread.)
Yet another, the Phantom hand cannon. The Phantom is a tough opponent that has to be handled carefully, but her hand cannon is overpowered so that most times you hear it, you're down already.
A general observation about the responses to this thread. I don't think that they can serve as a good guide to the game and its balance. The reason is that on the one hand, people who are in the top 0.1% of players clearly know the game better than anyone else and so when they offer their impressions and opinions, it's worth listening. On the other hand, being so much more skilled than the average, things that are fearsomely difficult for the other 99.9% of players may not bother them.