I believe the threads on a stock Fox NVA are a little loose, sometimes creates an air leak. I love my ole' Fox stunt .35, had it for years, very reliable. It'll do what it does, every time, just the same as last time. Just don't care for the needle.
Apropos of nothing, my very first stunt engine was a Fox .25, on a Flite Streak--that engine remained my favorite for years even when all around me were going to McCoys, Vecos, Maxes, etc. Kinda like a well-trained pet: It never bit me, always did what I asked, absolutely reliable. Nothing fancy, powerful, pretty nor fast--just always there, ready to go when I was. Wish I still had it (can't recall what happened to it).
Will people still be running (and buying) their current modern engines 50 years from now? Who knows, but I wouldn't put my money on it. That kind of success is hard to knock.
--Ray