(1) Because there was no cooperation between the decoders and video cards, playing HD 1080p content at 60 FPS would skip a lot. Perhaps a beefier CPU would help, but the same machine running Windows had no trouble.
(2) Samba, or at least Ubuntu's version of it, is very flaky. Some days it would see my networked drives, other days it wouldn't.
(3) Moonlight does not handle DRM content from Netflix. If my TV or XBOX did not already handle Netflix, then this would have been a show stopper.
All that said, I did run it that way for 6 months, so I guess it was "good enough".