One of the several scenarios in which a screw doesn't form a nice and tight seal on drywall paper just below the surface is if a joist/stud is not in the same level as the adjacent two and there is a gap between it and the drywall. As you're driving the screw to just below the paper, the paper breaks and the screw goes in too deep, resulting in less clamping force than a snug screw.
Q1: Is a screw like this completely useless? If not completely, how useless is it?
Q2: What percentage of screws per sheet are safe to be like this?
P.S.: I would appreciate no perfectionist type responses like "why do you have uneven joists" and such. I'm remodeling a 110 yr old house and I'm not replacing joists, some of which have sagged. I sister what I can but some require planing, and planing ceiling joists and having sawdust fall over me is my definition of misery. So I'm trying to be in an acceptable ballpark. Thanks