IMO, the best feature of FOL is the Ancient Forests. I genrally beeline FOL (especially when playing one of the elven civs) and let the Ancient Forests spread as much as possible. Once I have enough Ancient Forest growth, it is time to switch to a different religion. All of the Ancient Forests stay, and I can use the benefits of the other religion. I would recommend having both religions spread far and wide before the switch. To be honest, I think OO makes a better 2nd religion for the Svartalfar than CoE, but then I haven't played CoE enough to find its strenghts (if there are any).