yes. aluminium is much more fatigue prone than steel. aluminium is good for strength to weight ratio, but has no endurance limit so, no matter how low the stress, there will always be a finite fatigue life. steel can often indefinitely endure cylic loadings of approx 1/3 of its max stress. steel is easier to design for and get repaired.
as for the XR650R - the frame looks good, but, IMHO, the subframe should be remade in steel. the suspension is great, but fuel consumption etc may drop it from the top 10 overlanders. also its got a 1,000 mile service interval and its a bitch to start when the decompressor gets stuck.
all being well, I'll be able to compare XR650L, XR650R and Tenere when we get back from Algeria at the end of Jan.