I do not know what a Nuvista is so I hope someone hear can help you with that I do know what a Nuvistor is but it would not seem to be part of your original question it would appear because a Nuvistor is a vacuum tube.
Nuvista Tubes - General
May I have opinions please on the pluses and minuses of Nuvista tubes versus vacuum tubes? Supposedly and from the basic homework I've done, Nuvista tubes purportedly offer high reliability, low microphony, low noise, consistency, and small size compared to their vacuum counterparts. So what's the down side?