Would a pistol caliber carbine build be a good candidate for a polymer lower?

No.  Blowback uppers are even harder on the gun than gas operated rifle caliber uppers are.

I'll +1 this. About 5-10 years ago, back before ramping a 9mm carrier was a 'thing', it wasn't uncommon to hear about pins egging out the holes in an aluminum lower.


Also, I'll add that I used to have a complete polymer New Frontier 5.56 lower w/poly FCG, never had any issues on a 5.56 or 7.62x39 upper. One day I decided to swap it out with the aluminum lower on my RRA 9mm AR w/ramped bolt (I already had a blowback hammer on it for a .22lr build) and for some reason the poly hammer would never reset. It would reset when manually charged, and the carrier was traveling all the way back, but for some reason it just never worked right. But that same lower setup eventually ran fine on a dedicated .22lr upper.