An old bike won't be any harder to fit you to than a new bike. But whether new or old, if the frame size is wrong for you, then it can be a expensive challenge. Older components and some older frame standards might make replacement of them difficult and expensive on some old frames, yet some frames back then are easy to upgrade. So it depends!

You might like certain aspects of newer bikes, such as the newer STI's on a road bike and short reach shallow drop bars, compared to the longer reach and bigger drop of older style drops.

For anything other than road bikes, someone else will have to say.