They're not proper gbbrs in the traditional sense, they use a GBB gearbox from GHK. Think of them as a 'my first gas gun' rather than anything serious. Get a WE if you want a cheap gbbr
The M4's were designed to be AEGS, not to handle a shaking gas gearbox. They eventually shake themselves apart unless you thread lock every screw on the gun, but that would lead to problems as well.
I once had the exact same idea as you. I wanted a cheap gas gun so I picked up one of those CM16 GBBR's.
First, the magazines were horrific, they would misfeed, jam and leak gas.
The firing pin would get stuck half way sometimes causing me to have to rack the charging handle again. The gun, like Monty said, would quire literally shake itself to pieces.