I should add a caveat to this:
Guns with clear lowers are not automatically bad guns. Many platforms (especially ones on sale) offer a great platform for gaming and upgrading.
One in particular (the G&G RK47/ Top Tech line) is a great platform for gaming and upgrading (if you're down with TM standard breakdown).
The only real issue with clear lowers is that you should be prepared to find an alternative from the time of purchase. The failure of your clear lower may be years away, or days away, but it *will* happen.