Then why do you need any help?
If the circuit provides negative feedback at DC and has insufficient phase margin at some frequency, then slapping in an inverter will give you positive feedback, with some _very_ peculiar results. If you're using an XOR or other mixer for the phase detector then it'll invert automatically, and you won't be changing the gain/phase relationship of the loop.
No, a few more than that: Passive, single ended filters. Active filters using op-amps, inverting, non-inverting, and double to single-ended. Closed-loop control systems using several different types of microprocessors, DSPs and DSPs roped to FPGAs.
I haven't used pneumatics, mechanical amplifiers, magnetic amplifiers or dynamotors yet, but you never know.
From what population of PLLs are you drawing this "most"? If you're working with the current crop of PLL-based synthesizers-on-a-chip then using an inverting filter means you have to reverse the sense of the VCO command/frequency relationship, or you need to flip a bit in the phase detector someplace. If you're using an XOR phase detector then the point is moot, of course.