Per OPs question: another (older) use for nuvistors was for the front end of VHF receivers/preamplifiers and were employed in my (early 60s) ham radio equipment. The smaller size meant lower inter-electrode capacitance and therefor higher efficiency. Reliable low noise transistors for higher frequencies were rare and expensive then. Other than that (as already posted) they are just vacuum tubes. Thanks.
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?