Delixe: What exactly do you expect from AI? If you use stealth and take out all the soldiers without alerting anyone then yes it is easy because that's the point of taking them out. If you miss one or flub a kill someone will fire off a flare and reinforcements will arrive.
Well what I expected from the AI was to work like you said, that would have been the minimum ... except that in Crysis 2 it doesn't work like that at all.
Flub a kill, they will fire off a flare, raise the alarm... and... nothing, it's not any harder, maybe one or two new guy will spawn but as they are as easy to kill as any other it's not an issue,
In the end finishing the whole level completely stealthy or start firing rocket at random, doesn't change a thing. Just hide behind something for X seconds to let your energy recover, re-cloak and continue like if nothing happened.
Delixe: If you cloak in plain view of them they aren't fooled and will continue to fire at you. If you cloak then move they will search your last known position. If they are alerted (Yellow) they won't be fooled by you moving around in front of them while cloaked.
Actually they are, if you cloak in plain view they will shoot exactly at the place they last saw you, so just move one meter to the left or right outside the line of fire and you are perfectly safe (as long as you don't start shooting at them). And they only investigate your last known position is it's accessible and close enough otherwise they just stop forget about it.
In the later level I no longer cared at all about being detected or using sniper scope, so I walk cloaked until close to an enemy, de-cloack, fire with a short range weapon, re-cloack, and walk away, as you haven't shoot cloaked you you normally have more than enough energy to walk out of harm way.
The only time it doesn't work is with alien because of their rushing ability, in this case when you are noticed just activate Maximum Armor, shotgun or grab any alien foolish enough come your way and run to the next cover.
Also I never noticed any real change with either yellow or red mode, yes the enemies move, sometime they even throw a grenade at random, but it's as easy to grab them or stealth kill them.
EDIT: IMHO what they could have done that would have made it more interesting is that when "spotted" some enemies would equip themselves with infrared goggles, forcing you to be extra-careful as stealth wouldn't be as effective. They could even have included some infrared using soldier as "special" enemies guarding some key areas, it would have forced you to think of some alternate route/strategy to avoid them or neutralize them.