I’ve tried them, built an upper around one, and was less then happy with the outcome. Not saying the 15-22 is better, but it can’t be much worse.
I've trained with them in the USAF. The issue is accuracy, or the lack thereof.
A dedicated .22LR upper like the M&P15-22 has a true .22 cal barrel.
The .223/5.56 barrel is just a bit big for .22LR, so accuracy suffers when using the CMMG drop-in conversion bolt on a 5.56 upper.
But functionally, IF YOU USE GOOD .22LR AMMO like CCI Mini-Mags, they function just as well as my Ruger 10/22.
If you use crap "bucket-o-bullets" .22LR, you'll get crap results.