You'll most likely find 15mm linear rail has a higher load rating than V bearings.
15mm rail is more than capable of handling most CNC machines, it's just larger rail is a bit easier to mount (15mm typically needs M4 bolts, which require more care than M5 or M6).

I've just checked the load rating for some 15mm rail, and it is around 10kN depending on the carriage used. That's roughly the same as sitting a tonne on each carriage. Off course that relies on the rails being suitably mounted.