I have built two .22LR trainers. One is a service rifle trainer on a dedicated lower and matches my White Oak service rifle pretty closely. CMMG barrel and bolt. I use it for practicing the 100 yard reduced courses. Works flawlessly for this purpose. It's a little heavy for general plinking, but is also used for that. Saves tons of money. I use Bulldog mags in this one.
The other is a complete upper I bought on sale from PSA for grins and giggles. It is an 8 inch and wears a Warrior suppressor. Boonie Packer mag block and S&W mags for this one. It gets shot, a lot. I generally run the Remington Golden Bullet bulk pack in it, but also use Federal Auto-Match bulk packs in it. I'm not going to tell you it doesn't hiccup occasionally when a youngster is shooting it, but I rarely have any problems. The only problem is that even though I have never seen an ATF agent at our range, I have to swap off an SBR lower because I don't do the pistol brace workaround.
For my uses, the dedicated uppers have proven to be the best choice. I can also see that pressing the easy button and buying an M&P makes sense for a lot of folks.
If I was contemplating going to go to the expense of buying a commercial AR 22 and going to the expense of changing parts to match a rifle I already had, I'd definitely go the build your own dedicated .22 trainer route and end up with 7075-T6 receivers that will accept any mil-spec parts.