"Stamped out a lot" in that the receiver is made from sheet metal. I'm guessing before the ban in 86 they stamped out a lot of them as they are easy to make and registered just the receiver rather then a complete gun (either way it doesn't matter as it's still a machine gun). Group Industries did this with the Uzi receiver. Those receivers were eventually bought up by Vector and built into registered receiver Uzi's. They sell for about 3K. MP5 and AR-15 conversions are harder to find and as you said more desirable and as such are more expensive.
I have no personal experience with a Mac, but I have heard they are not all that accurate at long range. Of course they weren't designed for long range shooting, but if you are in a pinch and need it for long range you are kind of screwed. I wouldn't say it's a POS, but it is of limited utility.