The climbing here is unstable and dangerous; most areas are fenced off. Especially after a major a rockfall. This area would take a massive effort to make good again. Whether this is worth considering, given how much plant life has growth there is, is frankly debatable.
The Nantlle Valley is the forgotten area of slate climbing, almost totally ignored by the Slateheads save for a dedicated few who explored these holes and found a small set of rather adventurous routes. Ray Kay's The Purple Tailed Love Fish was a classic of its genre, but has sadly fallen down. All that remains now across the various holes are a few forgotten trad routes. Some of these look like they might be worth repeating, but in the main these quarries are something of a unknown backwater. They may well yield some exciting new routes to anyone wanting to explore. While some may well be climbable on trad gear, turning up with a drill and some bolts would probably help create more interest from other climbers. Also, some areas look good but would need lower-offs way below the top of the crag to avoid deadly top sections. An additional drawback is that many of the holes are flooded and require an abseil to a ledge or hanging belay to start the routes.