How do you - Yahtzee and some posters - expect a tranquilizer to work? Deus Ex was kind of cool in that a tranq dart is (A) felt by the victim, (B) takes time to knock them out, and (C) they can't be easily revived. Sure, that means it's not always the easiest weapon to use, but it's still "easy mode" in that game. Lean out, pop off a shot, lean back before it even reaches its target, back around a corner, wait for them to pass out. Repeat as necessary.
In DX:HR, supposedly a prequel, they've managed to make a tranquilizer dart that the target doesn't feel anything whatsoever upon being stabbed by a fast moving sharp object that sticks in their flesh. This will knock them unconscious through any ensuing activity unless somebody waves a hand in front of their face, at which point they immediately wake up. At least that adds some challenge to the whole thing.