Why FM receivers exhibit the capture effect: the strongest signal on a frequency takes over the detector and completely suppresses weaker ones, so you hear only one station even if two are transmitting. With SSB, two or more signals on nearly the same frequency all come through together and the operator can pick out the weaker one by ear. That makes FM a poor choice for crowded weak-signal work, while its wide 10-15 kHz bandwidth and capture behavior are fine for repeaters.
Watch out Voice quality is actually one of FM's strengths, since the noise-limiting detector gives clean, natural audio; SSB sounds thinner and needs careful tuning. FM is also easier to tune, not harder, because a small frequency error does not shift the pitch of the recovered audio.
FM "captures": strongest signal wins and the rest vanish. SSB lets you hear them all fighting it out.