G19 - no problems with old and new internals
G21 - no problems, new internals, is my favorite Gen4
G22 - no problems, new internals
G26 - no problems, old internals
G27 - no problems, old internals
G34 - no problems, new internals, "-" connector
G37 - no problems, old internals
About the only info I can offer up is that I chased down the newest RSAs and the new ejector for the G19, even though the pistol functioned just fine. The older Gen4s didn't have the "." connector, as do the G21 and G22. Basically, I am not fiddling with anything (because they currently work just fine) until something doesn't work properly or just plain wears out from use. Poor G19 rarely sees any range time as of late due to all the problems and part swapping that's been going on. I haven't fired anything through it since swapping out the ejector, but have fired some with the newest RSA. Didn't want the same thing to happen to the otherwise perfectly operating siblings.
Also note that the G21 was probably the best of all the Gen4 models I've bought that felt the best to me, although I'd say that the G34 is very close, but not with the lighter connector. I am thinking of putting a NY1 spring to match it (same config as my old G23 Gen2) so that the trigger reset is a little bit faster/stronger, but basically providing the same amount of resistance as a stock trigger.
Shooter of many Glocks
Ammo by Dillon and Hornady