Maybe, on my three computers at least (three different ones) I can boot from my USB drive without any problems. Might depend on what distro too I think. I currently have DreamLinux, LinuxMint and Ubuntu installed on it and all three start on all computers without problems.
Might also be beginners luck
since I have never before used a GUI, only servers (mostly Debian). Now I'm looking into some GUI distros to migrate all our Windows workstations gradually to Linux.