I wouldn't call the M9 a problem camera per se, but it had a few issues like shutter blade oil on the sensor, the rotting sensor, and occasional freeze/errors. My M9's sensor was replaced through Leica's program in 2017, and has frozen with an error code a few times over its lifetime. I do love my M9, though.
My understanding was that the M9 had a serious and quite widespread problem with sensor corrosion which was fixed by Leica until their stock of spare parts ran out. Is there a way to find out whether a given M9 has had the sensor replacement? (I think I remember you saying that you bought your very handsome M9 refurbished from Leica, so this is presumably not an issue for you…). I’m currently looking at second hand M cameras and the M9 would be on my list if I were certain that it had the sensor fix.
Ok, nice discussion, but lost the OT here... and, I think now there are sub-forums for the Leica gear, so this may get more helpful responses on the Leica M sub forum.
I'm curious of the answer, but have no experience with the Leica M system (tempted, but haven't dived in yet - I've held myself back with a Q2 and SL2)
I hope others can jump to help!
I, like the vast majority of M11 owners, haven't had a single issue with my M11s. The internet, forums specifically, are the extreme minority of owners/users of products. Even on line, only a handful of individuals have had ongoing issues - that, as far as I've seen, have been handled appropriately by Leica. In fact, the only threads I see about M11s having issues are perpetrated by those who have never owned one.