Funny we just put up new cabinets in a house my brothers and I inherited. As noted sometimes you can only hit a single stud, so we put 3x screws in there (full plywood backs, no particular hanging boards). 2.5" is sufficient and reduces likelihood of hitting a wire that's routed through the middle of a stud. GRK torx drive cabinet screws.
And yes connecting the faces is also proper practice. Clamp them together, pre-drill as long as you can, and run those same cabinet screws from one to the other. Carefully shim in the back and you can get them nice and flush.
The other trick (I'm not sure it's really a trick, just good practice) is to locate the studs and then pre-drill your holes in the back of the cabinet to match. Turn the screws in a wee bit, get the cabinet up, and then run a couple of them in not quite tight. If you did it right they'll be in the studs and now it'll hang safely while you get it level. Then tighten 'em up.
And I'll throw in a recommendation for small tools - been using my Makita 12v driver for this job, a larger tool simply would not fit in many instances, and in every instance would be bigger and heavier than necessary. Smaller is better for this sort of job
jeff