I dunno, the autopilot is certainly better than it was in 1988.
Looking at the update list, pathfinding AI is one of the things that was improved. Cant speak to how it was originally, but it was one of the few things the RPS guy had a beef with.
You tell the guys where to go, if one should follow another, wither to be offensive or defensive, and they basically do it. They'll probably cock up a full invasion, but you can send them back to the carrier or along a fairly safe route and they'll be competent enough to deal with any respawned infantry along the way.
Walruses aren't bright enough to avoid muddy patches where their wheels might slip, so they wont be as efficient under the AI as when you're driving. Probably not unexpected, though. They generally wont get totally stuck.
Funny thing, I once high-centered a walrus on a ramp and thought I'd have to write it off -- but got it unstuck by ramming it with another walrus. The AI wouldn't do something like getting stuck like that, but even if it did it would be fixable.
Mantas are pretty fragile, but they always were and they should be. If you're running a recon flight, you'll want to do it your self so you can hide behind the crests of hills and such -- those kinds of evasive maneuvers are a bit beyond what you can expect from the AI.
There are definitely some flaws. I wish I could decide which units to follow me, but as far as I can tell I can only issue the "follow" order to all nearby units. I wish indirect fire weapons had some indicator (ladder sight, that sort of thing) to give you an idea where it will land. Mostly it is fun, though.