I bought 2 new production SVT magazines a while back.
It took some dremel work (slow and patient) on the mag catch lug to get them to seat properly and lock in place well.
Then took them to the range and...
They just wouldn't feed. The bolt would go home without stripping a round off the top.
Back to the shop, and I looked more closely at the mags and the way it was seated in, and compare it against my 100% reliable mag that came with the rifle... The retaining lips on the top of the mag were bent too far over, which was keeping the rounds from coming up high enough for the bolt to catch and strip them from the mag.
Getting them bent into the right angle so that:
a) The top round came up high enough to get stripped and driven into place by the bolt
b) Not so loose that the magazine was self emptying
Was... A serious pain in the @ss.
Eventually, I got it to work. And my aftermarket mags now work as reliably as the original.
If you search around, you'll find that most people have similar experiences with them. The people who put the tinker time in to get them to work reliably are probably in the minority.
If these are coming from one of the existing production outfits that puts them out once in a while, expect a similar experience, and you'll pay roughly $80/mag to have that kind of fun.
If you have the time and patience to tinker with new production mags, go for it. Just don't get your hopes up for something that works "right out of the box" - because they seldom do.