I'm sure XTR is made for being used, but if LX has a steel axle and therefore space for bigger ball bearings than the XTR, there might be an advantage for LX hubs in this regard. It's the same problem with Hollowtech bottom brackets which don't last as long as cheap square taper ones which have more space for bigger ball bearings.
Your 4 hubs that have no issues don't tell us anything about the durability of XTR hubs in general. To estimate the actual durability we can only look at the design specifications and maybe some user statistics (as in returns etc)...