Most variables of the upper scale are pretty reliable, but of course, more moving parts = more to screw up at the worst possible time. Most of my pure "hunting" rifles wear the excellent Leupold 6x42. Never had one fail.
That being said, I had a 6x36 that had something come loose inside after many many years on hard kicking rifles. Strange part was, you could hear something rattle when you shook it, but it still retained zero, and adjusted OK. Sent it in for warranty, and received a brand new one, so don't know what the issue was. I personally prefer the fixed power scopes, but I would not feel hard done by if I had to use a Leupold 2.5-8 or 3.5-10. There are others that seem quite sturdy as well, but I usually buy those I trust from personal experience. Regards, Eagleye.