I think my main concern with aluminum vs polymer is that polymer is very rigid, where as aluminum can bend if you torque it enough, i.e. drop your weapon. Also polymer doesn't rust

I would support aluminum reinforcements or even an aluminum rail at the least.

I think if I wanted a metal .22 AR I would get a CMMG or even the SIG 522. Polymer lowers are a non-issue, perhaps even with 5.56mm ARs (There are a couple companies making mil-spec polymer lowers).