What types of enemies can't take cover?
Solution 1:
So far, I've found that the following types of enemies are unable to use cover (presumably because they're too big, or because they float/fly around all over the place):
- Mecs
- Archons
- Sectopods
- Berserkers
The following types of enemies also cannot use cover (as pointed out and confirmed by Chris Hayes in the comments:
- Gatekeepers
- Faceless
- Chryssalids
- Andromedons (after they revive)
- Psi Zombies