I see no problem with this lower if you want to do a dedicated .22lr build. The poly buffer tube won't matter because you won't be using a buffer. The only potential problem I can think of is the poly hammer not having enough force the reliably ignite rimfire ammo.
For anything bigger than .22lr though, I'd pony up a few more bucks and get an aluminum lower.
For anything bigger than .22lr though, I'd pony up a few more bucks and get an aluminum lower.