Maybe not lazyness, but not that smart either. Working with individual brushes costs a bit more time at first but it makes your levels much more manageable later on. I'd say, get yourself a habit of not hollowing but making the desired thing out of individual brushes. I nearly never hollow anymore. It's fast, yes, but doesn't give me the flexibility I need.
As for your grouped brushes, the hollow tool isn't that smart. It can only hollow each brush on itself, not the group as a whole. It's a quite limited tool...