Youtube clearly work hard to make their Flash plugin work everywhere. Despite other problems with the Google-owned service, for playing video, Youtube wins hands down. If it ever fails, then the blame is clearly Flash throwing a fit.

Vimeo, on the other hand, I have often found to be flakey. Only in recent months have I seen it "okay" in 32bit Opera on Linux, but the UX still leaves a lot to be desired. It has never worked for me on 64bit Opera on Linux.

Wade.