The boots can't handle the RPM of the driveshafts.
I did a search for RCV problems the other day and a link to Pirate4x4 was very informative, with additional links within the thread that I clicked open. No one there is bashing RCVs. Well, that's not true. They're bashing the shit out of 'em on the trail, and the RCVs keep on rollin'. High speed desert, monster HP crawlers...things that couldn't possibly be legal anywhere but in competition.
What do they have to say about RCVs?
Don't over steer them.
That's the only thing they have to say that could possibly even be remotely construed as negative.
Guys are blowin' out the cages, twisting shafts, whatever... RCV stands up with their warranty, no questions asked.
These guys go on to say there's no way in hell any other shaft would have survived, either, and more than likely would have broken many more times.
Bang for the buck? That's your call.