lighting - the more the better. MH preferred, vho ok, friend has a large derasa under t5s. Depends on the species as well. Have heard of them being kept very high up in the tank under PCs.
difficulty - in my limited experience, easier than sps corals, but not foolproof
special needs - if you're watching tank parameters like you should be (admittedly I'm not) you shouldn't have problems. Large ones devour calcium. Small ones may need fed (filter feeders, phyto, etc), believe the cutoff is somewhere around 2.5", depending of course on the light? Some fish will nip at the mantles. My lawnmower blenny does not.
Just my general take on clams, much more knowledge to be shared...