Blowback pistol ARs are not the most pleasant gun to shoot, as you have a heavy bolt and buffer bouncing back and forth. Recoil is more than some folks may expect. That said, a blowback AR is a fun project, though I would want a longer barrel — maybe 8-10 inches — to get more velocity than my P97DC generates.
The main pros of a PCC AR, as I see them, are:
1. Can practice with an AR at ranges that don’t allow rifle calibers.
2. During normal times, ammo may be cheaper than 5.56mm.
3. Pistol calibers are easier to reload, and blowback ARs can use (cheap) cast lead boolits.
4. Less blast is generally helpful when introducing beginners.
5. PCCs suppress much better than rifle caliber pistols.
6. May be faster and more accurate than a holster pistol, depending on the shooter and how the AR is set up.
Some cons:
1. In absolute terms, is less effective than a rifle caliber.
2. Some blowback ARs aren’t all that pleasant to shoot (a 3-inch .45 ACP is probably in this group).
3. If your budget is limited, there are likely better choices in an AR.
I don’t hate pisrol-caliber ARs, within their limits, but a 3-inch .45 isn’t for me.