My favorite 9mm AR was one I built with a Cav Arms lower. I used a RRA mag wel adapter which once adjusted ran 100%. That rifle was well balanced, light, and fit well. I sold off some of the upper parts but kept my Cav Arms lower. My only regret was not buying a couple more colors. Maybe I can add a FDE one later...I love the Cav lowers.
Do NOT compare the NFA or ATI or other plastic lowers to the Cav Arms (I now...they have a new name now...but I am old and just know Cav who developed them). The Cav Arms lower was designed to be made of the material it was produced in. They didn't do a copy of a lower designed for metal as they are two different materials with different properties. I do run a ATI Omni lower on a budget .22 build and for that use its OK. But NO WAY would I build a 9mm on the Omni, I think it would not withstand the beating. I could be wrong but ..... I don't want to take a chance.