Well, I can't really speak for Gentoo because it's not really my cup of tea, but XAMPP runs fine on Ubuntu. Granted, some problems come in here or there, but there aren't too many things that I haven't been able to figure out.
One problem I've seen Ubuntu users encounter, but I've never personally encountered, seems to stem from using more recent versions of Ubuntu. I personally stick to using 6.06.1 LTS for production server usage.
The other problem I encountered had to do with a recent upgrade of XAMPP. The specific problem pertained to the backup not completing successfully. It'd fail at the phpmyadmin stage. I simply rolled everything back to the previous version of XAMPP and will wait for one of the 1.x.xa or 1.x.xb releases of XAMPP.
I wondered why the frisbee got bigger and bigger. . .then it hit me