I don't think that's what the criticism means at all. But to address this point, I can't really say much about precedents with stealth games because I don't play them often. I have some old-ass Splinter Cell games on my shelf and that's about it. Although I seem to remember games like Blacklist being pretty challenging, but maybe that was because I was younger.
My criticism/complaint is a two-part critique that I have not just towards MGSV, but towards many, if not most games. And that is first of all, the the horrid state that AI is currently in. It's not improving and it might not for some time. Second, the difficulty curves in games, where we start with great balance, and then by the 1/2 or 2/3 point, everything becomes easy as shit. In MGSV, the first 10 hours were rewarding, tense and full of risk/reward measurements. By about 20 some-odd hours in, everything was easy as shit. Same thing happened to me playing TW3 earlier this year. By about 50 hours in (half my playtime), the game was stupidly easy, even playing on Blood and Broken Bones. I guess some people enjoy power trips, but I like to be challenged in games. I like a sense of accomplishment, and MGSV left me with none of that.
I don't think anything I'm thinking of is really that revolutionary though, in terms of AI scripting, even within stealth games. Some more CCTV cameras. More randomized patrols. More helicopter scoutings. More persistent base searches when the enemy is alerted to my position. A real manhunt doesn't end in like a few minutes, they should search you out actively until they find you or actually secure the entire building or base. Maybe more base modifications, like added barricades and such. Less deaf guards (I can fulton a tank while a guard is standing 20 meters away, really?) Less static placement of guards and weapon points since I quickly became bored because of the repetition of those placements. I mean, want to be really ambitious? How about enemies that have some more "dynamic" traits such as cowardice, cautious, reckless, etc. rather than just copy-pasted same-y enemies?
Unfortunately, my complaint will really never be addressed. The average consumer is way, way more interested in "all dem gwaphics" than being challenged consistently.