What's the difference in 'rollable' and 'normal' basecoat?
I'm not familiar with a "roller" vs "brush" primer. The only thing I can assume here is that is that they're describing "new drywall" (example) vs normal primer (example), where the former is thinner and cheaper.

New drywall is porous and tends to absorb more paint as a result. New drywall primer is a thinner primer designed to fill those gaps and nothing else (i.e. it won't block stains, etc). I've used it with rollers and never had a problem.