I own a lot of cameras. The only brands I have had no trouble from are Horseman and Mamiya. My Horseman 45 has never had a problem (admittedly, probably because there is not much to go wrong), and my Mamiya 7II has not skipped a beat. On the other hand, cameras from Leica, Hasselblad, Rollei, Contax, Canon, Fuji and Konica have all given me trouble at one time or another. I treat my equipment well, but the fact is that if you shoot a lot and use a lot of cameras, they are going to need work every once in awhile. For what it's worth, a lot of the cameras that I have had problems with are the ones said to be pinnacles of reliability and durability -- the Leica M3, Canon F1N, Hasselblad 203FE. The one that is supposed to be fragile -- the Mamiya 7II with its plastic body and "easily knocked out rangefinder" is the one that has held up the best, with much more rigorous use too...
All that said, I don't have many sleepless nights about cameras failing. If you are in a critical situation, you should have a backup. If you don't, you are just tempting fate. As for Leicas in particular, I don't think they are inherently more reliable or less reliable than any other mechanical and semi-mechanical cameras. For me, their strength is in their design and optics, not in their supposed indestructability. In general they hold up well with use, but they are still fairly delicate instruments.