While most all mags are made of metal, you do need to look for rust as even if it's made of stainless, it will rust if it's low quality or those mags that look to be coated steels. Mags are getting expensive for them to be considered a replaceable item since springs and followers is usually all they need.
I've only found 2 magazines, from 2 different manufacturers that gave me a problem. One was/is my EDC S&W SD9VE magazine, which was shipped with 2 mags, but only one had problems. I pushed a dowel down on the follower and saw the follower tilt and bind up some about 1/2 way down. Torn down the mag and found that where they had welded the magazine along the back side had a weld wire burr sticking out ever-so-slightly on the mag's interior. Removed it with one of my jeweler's flat file and corrected problem. Re-greased ever so slightly and works perfectly to this day.
Another mag I found to give some problems, but only occasionly was an older magazine from Hi-Point JCP40. The magazine's follower when looking at it while still inside magazine, I noticed the follower had plastic flashing on it. Took apart and cut flashing off, and also used my knife to give the bottom edge of follower a slight chamfer to eliminate the sharp edge. Works great to this day.
I especially watch carefully on my S&W I mentioned above as this is my concealed weapon, so dirt, lint and just everyday sweat get on everything, so I do very close maintenance of cleaning, oiling and greasing of it to make it work for those JIC situations that nobody wants to have, but never knows from day to day.