I'll take a stab at it. The main thing you see failing in servos is the gear train, in which case, they flop around loose or skip. Or I have seen several cases of linkage failure. Broken horns, broken solder couplings and in one case a broken clevis. This leads to the bad situation of floppy control surfaces and possible flutter of the loose control surface. I am not sure I have ever seen a servo that seized. Also seen a few cases of bad electricals that made the servo appear dead but was either connector or in one case broken lead. Once saw a melted extension. So the overwhelming number of cases I have seen are either loose or dead servo(as in unpowered). Not seized. my 2 cents.