My personal definition of "science fiction" is pretty narrow. So much of what someone else might call "mixing science fiction & fantasy" is to me "mixing fantasy with fantasy".
If I ever did get to play the kind of game that I'd be tempted to call "science fiction", then there's a good chance that any magic--even disguised as sufficiently advanced technology--is going to annoy me, except for maybe once. Since the whole point would be to limit it to things that were most plausible.
If it's fantasy up front, however, almost anything goes. Although, I still generally think it's best to be judicious with the fantastic elements.
If I ever did get to play the kind of game that I'd be tempted to call "science fiction", then there's a good chance that any magic--even disguised as sufficiently advanced technology--is going to annoy me, except for maybe once. Since the whole point would be to limit it to things that were most plausible.
If it's fantasy up front, however, almost anything goes. Although, I still generally think it's best to be judicious with the fantastic elements.