I really tried to like stealth, I really did, but I just couldn't. Some of my main problems are:
1) As many have said, the enemies always seem to spot you, even when crouched with the camo upgrade, over 200 yards away in a huge bush that's behind at least 5 cars...
2) After sneaking up on a guy who's alone and nobody near his buddies, I pull out the machete and knife him which apparently is the loudest thing you can do because suddenly the whole camp knew my exact location, I would have been better off shooting the bastard.
3) Even if you shoot a guy with a silenced weapon and his buddies aren't alerted, they seem to have a tendency to get really suspicious and always get a strong urge to all circle around and check out the bush you're hiding in. This really messes things up, because even if I'm rather successful at stealth, I can usually only get 4 or 5 of the guys if I'm lucky, before his buddies come sniffing around my bush.
4) The guard outposts, I mean you get all of your stealth gear and drive along to the target camp, ready to fight and kill people. But hold on, it seems some guards spotted you driving along and decided to follow you or the asshole designers put an outpost right outside the camp, meaning you've got no chance of killing people in that camp silently anymore.
5) And finally, the locations. You want to climb up on top of a nice cliff and snipe your target quietly before slipping away? Sorry, not gonna happen. Why? For some reason, the designers seem to have made the majority of camps a complete circle, surrounded by steep and unclimbable hills, leading to only one or two paths into the camp where guards love to hang around, with your target hidden right in the middle of a house, meaning you're not killing him unless your ass goes in that house.
There are many more issues with the stealth in this game, such as the MP5 not being quiet at all as well as limited ammo for the silent guns, but there's no point in ranting any further.