Not that I've ever built an AR, but my impression has been that the biggest downside to building one is the fact that you'll just want to run out and build another. And another . . . and another. . .
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman