I recently bought a P238, my first Sig. Part of its attraction was knowledge that MagGuts.com makes kits to increase its 6 round mag to 7, and its 7 round mag to an 8 round capacity. I didn't have any personal experience with that product, but the other reviews looked positive, so since I liked the controllability of the P238, I decided to take a chance on the combination. Bottom line - they work. I had bought two 7-round magazines from Reed's Outdoors from a recent sale, and two others came with the pistol. I didn't want to muck with my carry mag until after proving things out, so my test used 3 modified magazines.
The first few times loading the magazines has proven difficult in getting that last round into place. I have read elsewhere that this eases somewhat after a bit of use. Bottom line - MagGuts inserts caused no jamming or failures of any kind when using Precision One XTP or Fiocchi Perfecta JHP, in either slow fire, double tap, or ripping off the entire magazine.
The only trouble came with trying to use Kypu bimetallic rounds. 2 FTF and one FTE. These rounds were incredibly dirty, which I believe conspired with the bimetallic cases to completely lock up the pistol, ending my range session. And OBTW, at the incredibly long range of 5 yards, the Kypu bullets were tumbling, creating rectangular tears in the target. So, I am blaming the failed shots on the ammo, not the MagGuts.

I have used them on a Glock 42 and S&W 9mm Shield with no issues, I think is a good product.

I would never use them. I depend on long term, uber reliability. I'd rather have that, in factory mags, then always have a worry in the back of my mind that the aftermarket parts shit the bed. And I always carry an extra mag, so I don't find that extra round all that important.

Just because something is OEM doesn't make it de facto reliable. Just because something is aftermarket doesn't mean it isn't. Between testing it myself, and finding out what reputation it has, if the part proves out, I'd have no problem trusting it.