The only ones I've replaced are ones that have gotten very wet inside multiple times. Something that could have been prevented at the first sign of trouble. My personal machines have the lower steering shaft drilled and tapped and have a set screw installed, to remedy the play these machines develop over time. Improperly sealed connectors and corroded eps relays are common problems also, along with improper installation to begin with. I had one that pulled to the left no matter what... ended up being main harness improperly routed and pulling on the upper steering shaft, which the eps takes as steering input and amplified it. I've heard of all kinds of crazy things, but I've been working on them every day since the 09 eps came out, and these are the only problems I've seen with it, every one of which is preventable.
As far as the XP goes, expect to replace a-arm bushings and balljoints often. I have two XP's, over 10,000 miles between them, never an issue past wearable parts. Never a CV joint, as I am diligent about checking and replacing torn boots. Never engine, trans, or diff work. My 550 with 8200 miles has had one belt replaced, and wearable clutch parts replaced. My 850 has had a belt and lots of water proofing done (No doubt because the power of the thing tends to influence riding style a little!!) It's all about how you maintain your machine and the quality of the service center it goes to.
Honda's eat timing chains and the electric shift is so unreliable that Honda actually provides a tool in the toolkit to manually shift the machine when it fails. They are still great machines.
Yamaha's clutching will cost you a fortune to have repaired if you wait until they fail to have them looked at. Still a great machine.
They all have issues, and the nice ride of our machines tend to give people a false sense of security. I'll hit ruts and bumps and 50mph on my 850....wouldn't dare do that on a Honda, no wonder my suspension needs work first!
Buy what you like the best.