Retro mountain bikes

There are videos on youtube of people comparing modern (carbon/ full squish) bikes to older hardtails (riding the same trails/lines/etc) and the verdict is that they work but kinda suck in comparison. Any of course they get the job done, people have been throwing them around for decades but it all boils down to how 'gnarnly' you plan on getting. My Evil the Calling inspires muuuuch more confidence than my Litespeed Obed equipped with period correct parts.

The only real difference between a (decent) late 90s MTB and a modern rigid bike is worse tyre clearance and lighter weight. Put some decent rubber and gearing on them and they're plenty of fun in your average woods. Really depends on your aspirations.

Rode this for years as my only MTB

Mid 2000s is betterer though. You get disc brakes and room for a decent 100m fork if you so wish.

If you want to race, buy a modern 29 'er. I have a Ritchey P Team (26") with modern 2x11 xt and although it's a superb bike, it's frustrating when everyone leaves you behind (even the ones that have less skills and that can't follow e.g.on a road bike). But if you don't race, a decent vintage bike is as much fun as a modern one. When used in dry conditions, even XTR M900 canti's have plenty of braking power. When it gets muddy, Magura rim brakes or disc brakes are less frustrating though.

I'm wondering how well something from the 90's holds up now and whether its viable to use a retro mountain bike instead of buying something more modern.

Depends on what you want to do with it. Modern kit - both the racy stuff and the rad stuff - is light years ahead if you want to push yourself. If you just want to bimble around the woods with your mates and only get 1" or 2" off the ground early 2000s kit is plenty.