I don't know what to answer. I'm never detected when I don't want to be, unless I make a mistake. And stealth is a large part of my preferred playing style in the game.
If anything I'd say Far Cry 2 had one of the better stealth systems I've come across in shooters, and I'm hardly inexperienced when it comes to playing games with stealth either :shrug:
For the record, I play as a sniper mostly. I carry a silent sniper rifle, a bolt-action sniper rifle (because it's more fun than the other two rifles) and a pistol. My whole playstyle is almost directly opposed to how you say you're forced to play it. I usually try not to get into conflict until I reach my destination, preferring to sneak around enemies or eliminating them quietly. I usually try and scope out targets, eliminating outlying hostiles silently (a silent sniper rifle is essentially 8 free kills, as long as you don't miss and nobody sees you making them) and seeing if I can find or create a path to my objective. If so, I usually try to go for stealth. If not, I switch to my bolt-action rifle and it becomes an entirely different game

I've played through most of the game twice (Bugs got me both times), and I'm about three quarters through it now playing this way. I must have played it for around 80-100 hours total? I tend to take a lot of time, especially since I often like to walk.
Sniper rifles are for sissies. A pro uses the knife, where it's up close and personal.

A garrote would also be acceptable.

It's been ages since I played the game, as I bought it on release, and perhaps they've patched it since then (I recall reading that they'd reduced if not entirely removed the respawn at checkpoints, for one thing) but mostly what I remember is being completely frustrated by the "stealth" part of the game. I'm usually one to creep through the perimeter after knocking out a weak-point sentry, then completing my objective from the inside out while avoiding contact as long as possible - Rainbow Six style, if you will. However, even by milsim standards these guards were just too alert to pull it off cleanly, and no matter how careful or cautious I was there'd always be some gunner or sniper a thousand yards away who would somehow spot me creeping through the brush in the dead of night (without NVDs either.) I'm not asking the game to be realistic per se, but at least fair.
Splinter Cell, Rainbow Six, Operation Flashpoint, Thief, Deus Ex, Hitman, Crysis, STALKER - all these games have different forms of stealth gameplay, and none of them are as fickle as Far Cry 2 (well ok, maybe Hitman...)
It certainly fared a lot better as a straight up shooter, and as I said I enjoyed it a lot more when I wasn't trying to overcomplicate things. Ultimately it's the little things that really wear it down, though, like they didn't fully playtest the final result of all their good-ideas-on-paper: for example, instead of driving to your objective and starting your mission, you first have to drive for about a minute, run into an enemy patrol coming the other way and either ditch the vehicle and hope they didn't see you, or jump on the gun and play rock-em-sock-em with their gunner (it takes a real sadist to design a game with a gunner's seat
but doesn't give you a gunner!) then leap out to fix your engine, hop back in, drive through a checkpoint (I stopped killing them after a while and just ran through, no point), meet another patrol, kill/avoid them too, fix your engine again, repeat, and then
finally just as you're getting to your objective area
get hit by those damn malaria effects! :banghead:
Everything just felt like such a chore, essentially using all the worst bits of "realism" without the rewards to go with it.