We've had a couple of Retrobike meets at Llandegla over the past couple of years.
Both the red and black runs are pretty easy even on a fully rigid bike with canti brakes at Llandegla. The surface is very smooth and flowing there, so no problems at all.
If anything, sections of the lesser used blue route, which has a less smooth surface and flat, rather than bermed corners, is more of a challenge on an old bike.
Of course you can ride the trails there a bit quicker on a modern bike, but in general most of us had no trouble keeping pace with many of the 'modern' riders.
There are some Llandegla aftermath threads here:
And here's elPedro666 'railing a gnarly berm to the maximum radcore' dude.
Other trail centres vary, but in general most black runs can be tackled on an old bike. Natural riding is often much more technical.