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):

  1. Mecs
  2. Archons
  3. Sectopods
  4. Berserkers

The following types of enemies also cannot use cover (as pointed out and confirmed by Chris Hayes in the comments:

  1. Gatekeepers
  2. Faceless
  3. Chryssalids
  4. Andromedons (after they revive)
  5. Psi Zombies