I shouldn't ask this as am happy with my frames, but out of academic interest.

Have 2 oldish (Dekerf) frames, one designed around 80mm and t'other 100mm forks. Both can take disc brakes. Both originally designed for XC which is 99% of what I do (and 100mm fork is ample for me). Both made from lush material and beautifully put together.

So, will they be 'less good' than modern XC frames? And if so, in what way?

Ta

Not really

No. Depending on where you ride, could even be better to have sharper steering.

Possibly smaller strength to weight ratio?

What was the question?

For xc they are probably as good as, or better than, anything around now – unless you want to go down steep or steppy stuff.

I've recently craked an Airborne Lucky Strike. Very long, old school xc race geometry. During the same period I was also riding a Pace 305 and Anthem X.

The Lucky Strike was quicker everywhere except on techy downhills (where it was a very scary ride). Pace and Anthem are both far faster DH, but the Pace especially, doesn't even get close to the same climbing times.