It's not necessarily a bad thing to have anaerobic bacteria if the substrate remains undisturbed, but it can be problematic if the gases from the bacteria escape into the water column.
Oh yeah, I'm a big fan of 'hills' and sloped substrate as well. I do it in all my scapes as I feel it adds depth to the aquarium visually. I think I've been told the reason aquascapers use volcanic rock specifically is because it is quite porous, it doesn't allow as much anaerobic bacteria to build up beneath the surface of the substrate. It's not necessarily a bad thing to have anaerobic bacteria if the substrate remains undisturbed, but it can be problematic if the gases from the bacteria escape into the water column. Lava rock allows oxygen to flow a bit more freely and adds some minerals for the plants.