I have 2 Frontier polymer lowers that I have shot thousands of rounds out of. One with a 5.56 Noveske upper and the other with a 300 BLK CMMG upper. So far, no cracks or failures of any type. I have a third Frontier lower but I have not shot enough rounds out of it to mention.

Noveske upper on a Frontier lower.
That’s my Man, Inspector.

What Ren said.
Think about barrel, bcg and trigger.
Get an extra bcg and springs or lower parts kit.

Definite differences in accuracy from brand to brand and gun to gun.
Even same brand and model can vary from one gun to the next. Less variation with upper tier manufacturers.

Most will run and shoot fine. Going from the quarter to the dime is where the differences are made in barrel and trigger.
Just get one. Play with it then prepare to get another one.

(If you notice all the Monkeys here couldn’t get just one. Me included.)

Black rifle disease.  I had it bad at one point, and get relapses here and there. 

Now I got the bolt action rifle and reloading disease.  Just before COVID and ammo situation, I was getting back to shooting and taking classes quite regularly.  Maybe average of a few hundred rounds a month.  I prob shot less than 100 rounds in the past year.    

Noveske upper on a Frontier lower.
That’s my Man, Inspector.