Belly bands are great if you need to wear gym shorts, sweat pants, or even deep conceal where you can not have anything external show at all. Could also be an option for a BUG.
My concerns with them are safety. How well does it secure the firearm from falling out? How well does it protect the trigger from accidental discharge? Don't be that person on the news who dropped their gun or had an ND in a public place. A holster is not a good place to save a few bucks and go cheap.
I have a
Modular Belly Band from Crossbreed. This uses a kydex holster 'module' that attaches with velcro to the band, which then also wraps over the holster module to further secure it. The kydex holster holds the gun securely and the trigger is protected. Of course it's adaptable for different guns with additional holster modules.
I think Crossbreed's belly band is a good solution to the above problems, very safe and fairly comfortable. You do have to make it fit tight and it does get hot since it wraps all the way around you. It's not as comfortable as a regular belt holster so it is not my primary method of carry. It also works best with a light handgun. I only use it with my S&W Shield or my alloy frame J-frame revolver.