Very timely thread as just last week I took my 31 year old GT Avalanche out, which was also the first fully rigid ride I've done on a mountain bike since the early 90's. Like the OP it really focuses the mind and was a fair old workout, but it took me back to when this was the norm for us mountain bikers. I've fitted Smokes as well and I had forgotten just how damn good these tyres are.
I think that it's quite obvious that early bikes aren't going to be as fast or necessarily comfortable as modern stuff, but if that's accepted, the use of the bike is purely then for the pleasure of recreating the purity of a former time which can't be bad.
The durability of equipment of course depends on its former life and what material the component /frame is made from. I would think that NOS or very close to that will be as good as the day it was thrashed round the trails in the 80s or 90s. Just a case of applying a bit of common sense here. There's always a risk of failure, even with modern stuff I guess. I tank at full speed down road descent on my 33 year old Specialized Allez Epic with bonded frame and original aluminium forks and try not to think about that sudden snap!