Agree with you on the Observers and their hit boxes. With sniper rifle I'll have it lined up without moving and it's not uncommon for it to register as nothing.
The Breachers I don't mind too much, sure they can spawn a ton and behind you and surprise you but not too bad overall. I've noticed only lately how they have the ability to detonate from below you instead of just at the same level as you.
Destroyers are not too bad if you have decent cover and have them more isolated. Their shields are quite high, so I can see how the armor is lower as their creators built them to withstand attacks via shields and have worse armor. Not a total walking death tank with ridiculously high shields and high armor.
