The V12 has been around for a long time now so the teething issues are ironed out.
The V8 is the same unit used in AMGs, so it's it's absolutely bullet-proof. But it's not a V12
Like any smaller/exotic brand, servicing will cost more and parts will too. But if you stick rigourously to the service schedule you should be fine. I tend to do an interim oil/air filter change if I've been driving my car hard as well.
On a Rapide, pads and discs aren't cheap but they're not much more than I'm already paying. Fuel is a consideration but I only drive around 6k miles a year now. They're not a brand to go for if you like cabin tech, but who wants to drive along watching YouTube when you can listen to the roar of a V12?