External invincibility is a bit of a hack-job in that it's not an elegant solution, but it works for the purpose it's designed for (preventing running in and nuking the core command, or blobbing the AI eye).
Guardians are amazing because, among other variety, they've allowed for turrets to be balanced for the player's needs, rather than trying to balance them to make them not impossible to deal with when the AI has them as well. And, from my experiences facing the upgraded 4.0 turrets while the Hybrids were still glitching and building turrets, that would NOT have been a fun experience. Guardians also provide some interesting oomph to AI counterattacks - in 3.0 I used to run in, spank the command center, and run out, because I knew there wasn't going to be anything in the way of serious reprisal. Guardians, with the new AI waiting/coordination of threat behaviors, now allows for the AI to have some teeth in responding to player actions, not just in launching waves.
AI Eyes are, again, not an elegant solution, but they pose an interesting puzzle that I am happy to work with.
AI Core Shield gens I do not use, because the mechanic was poorly implemented and has no use to me because I've never been one of the super low-AIP deep strike players. Forcing the player to destroy each one individually, rather than having switch-on-planet-capture to allow for scrapping is a nuisance, and keeping track of another 25 objects that NEED to be dealt with is a pain (and yes, you do need to keep track of all of them, because for the C, D, and E generators, which aren't linked to any "benefits", it takes careful consideration to know which planet in specific, of each, is going to be the best to advance your goals).
Varying AI guard posts and core guard posts means nothing to me - the guard posts are either not threatening enough, or not varying enough (the exceptions are Spire Shield Gens, Counterwave Posts, Command Shield Posts, and Core Neinzul Spawner Posts) for me to really care about whether a post is a core electric, or a missile, MLRS or Arachnid, or whatever. Even the Core guard posts are quite easy to kill and do not pose any significant threat.