RD,
Quite a few. Along with Leupolds, there have been Zeiss, Swarovski, Kahles, Nikon, Weaver, Bausch & Lomb, Bushnell, Burris, Simmons, and some others I'm not remembering. (Of course, some companies don't make any fixed-power scopes at all--but I've seen their variables go belly-up too.)
I've had variables of all those brands fail (often multiple scopes) but rarely a fixed-power. In fact I can probably count the failed fixed-power scopes on the fingers of one hand--that is, those that broke through use and not abuse or accident.
Have owned some really "affordable" fixed scopes that held together on some very hard-kicking rifles, and some really expensive variables that fell apart quickly, sometimes on mild-kicking rifles. Maybe I'm unlucky (or maybe I use rifles more than some other people do) but after the same thing kept occuring, even in this age of supposedly trouble-free variables, I qit trying to fight it and started converting most of my big game rifles to fixed-powers. The only variables that remain are a few that have really proven themselves, such as the 1.75-5x Burris Signature Safari on my .338.
Of course, there are some fixed scopes that just don't make the grade due to other problems. The 6x40 Nikon is one of those, at least for me, due to a combination of a short tube and relatively short and critical eye relief. The Burris 6x40 Fullfield II, on the other hand, seems to be one of the great bargains in big game scopes.
