Don't waste your time or money on a polymer lower.
They tend to break where the buffer tube threads onto the lower.
There are some hybrid polymer lowers that are reinforced with metal.
Lowers don't get a lot of the abuse, but that being said, an aluminum lower is going to be much stronger.
Why do you think so many companies offer 50BMG uppers that mate to a standard AR15 lower. These are the "salad days" of inexpensive AR15 parts. No reason to get a polymer lower when you can get a forged/billet for not much more.
Most guys that have them and use them for hunting, target shooting, etc, and not for pry bars and boat poles, get just fine use out of them. They are lighter than aluminum, and serve the purpose just fine for lighter-weight, general purpose builds. A few early designs left a bit to be desired, but current models seem to work just fine if you're not swatting zombies over the head with them......same tends to go for a finely-stocked English straight grip 20 gauge. Check the reviews on them - JoeBob's wont sell problematic junk. The job of the lower is to simply hold all the pieces together in alignment, and to some extent, absorb the recoil of the mighty 223....poly does that just fine.
That said an aluminum one weighs about a half a pound more, and costs about $40+ more than poly complete.