If they’ve only done towpath pootling before, the Cwm Carn climb will put them off Mountain Biking for life! To those of us seasoned riders, it’s quite normal, but Cwm Carn is one massive climb followed by one massive descent.
As for the FOD. Well you can get lost if you don’t know where you’re going indeed. But there are 2 waymarked XC loops there, both of them ideal for getting relative newbies to the sport into it. They’re fun ridden either fast or slow, and appeal to people of all abilities whilst they’re still fresh to you. So you can have a good time there without getting lost. Though there are hundreds of miles of quality singletrack away from the beaten path if you want to find it! Probably best saved for another day though…