Like other engines where the pistons rotate (e.g. Gnome) it does have the weight advantage of not needing a flywheel (since the engine is its own flywheel). Other than that, probably not much.
Without seeing what engines they are comparing it to, I can't say for sure. My guess would they are comparing at similar RPMs, or have some other restriction. I'm sure it weighs a lot less than a traditional 3L engine, but I would guess it generates a lot less power at redline.
Sure its different, and comparing it via the normal means (litres or rpms) won't work because its so different - those means assume a substantially identical engine or they don't work at all.