Ehhh I would say the amount of actions AI can do is important, but general AI in most stealth games are not good at all even in some of the best ones. The popular reason people give for that is because the more realistic the harder the game would be and unlike a shooter you can't just shoot your way out, you have to outsmart your opponent.
Yeah MGS games haven't been at the top of the stealth genre since the first one maybe. Really I would argue the MGS series has always been a 2nd or 3rd tier stealth series, the original metal gear games were the ones that were waaay ahead of their time. But around the time MGS came out to the time MGS 3 came out we had series like Thief and Splinter Cell arrive and blow way past what MGS was doing for their respective years.