You're using 'stealth' wrong if you want to use stealth gadgets in encounters with a truckload of bad guys :-P They're designed to take out a few opponents that happen to notice you, not as direct tools of confrontation - that's what the pistol is for.
Seriously, stick to high ground and you most likely won't even get noticed, let alone have to take out guards. Alternatively, just drop the 'Low-chaos' pretense and mop the floor with everybody, if stealth is quite simply not your thing, there's no reason to torture yourself over an ending slide.
Some of the maps simply don't have a lot of high ground or sneaking through spots. For instance, that dude's mansion/laboratory. There's a bunch of guards roaming the place, but as soon as one even notices you, those stupid robots come running out and all stealth goes out the window.
I also don't get why there aren't spare sleep darts laying around. I get conservation and all that, so they don't want to drop you 5 darts each room you enter. But the entire mansion and there's not a single sleep dart? The loot drop doesn't exactly encourage you to go stealth, which is my issue.
I know there's no rule that says you HAVE to go stealth, but it just feels like some of the maps go out of their way to punish you if you do attempt to.
You do realise with the mansion/laboratory you can literally get yourself into the walls?