While my experience with the USGI-type aluminum 20-round magazines is quantitatively better than that of OP's, it is qualitatively similar. I believe he is being honest about this, as I have had some recurring and systemic problems with that style of magazine, that I have not with any other magazine designs.
Various good designs of 30-round mags have been flawless for me, while I have regularly encountered 2 issues with the 20-rounders. These problems are equally common with very old and very new magazines (whether used or new-in-wrapper from the '60s or today), and are not ameliorated at all by installing new springs.
The first issue is that across various dates of manufacture (from 1961 Colt/ArmaLite-marked to current-production Okay Industries), I often, across 6 different rifles that never encounter this problem with any 30-round mags, see these magazines fail to lock the bolt back after the last round. It seems a bit more common with the mags with the aluminum followers than with than with the plastic ones, but the new-in-wrapper Colts from the 1960s do it every bit as much as the ones that were clearly used a good bit before being surplused.
The second issue is that, in my experience, the USGI 20rd mags do have a weak failure point that is not present on any of the 30s I have tried, and one which this C-Products design looks to fix. This is that the rivets that hold the steel strip into the front of the magazine break easily. This has happened during regular competition, field, and range use that does not phase any of my 30rd magazines, but most often occurs when I take the magazines apart. The part of the steel strip that bends down and latches into the floorplate puts a lot of leverage on the bottom river, and when the latch is depressed, even when I am being specifically careful to go easy on that rivet, the rivet regularly breaks, essentially making the magazine permanently broken. This has occurred both with very early-production Colt/ArmaLite magazines, as well as new-production Okay Industries mags.
While I have no personal experience (yet) with the 20rd steel duramag units, they look like they may fix both of those problems with the self-levelling follower and the different floorplate. Also, my experience with the C-Products stainless steel 30-rounders (all made 2016 and later) has been excellent, and they are some of my favorite mags. I know C-Products had some serious issues back in the late 2000s, but in my experience (I have A LOT of their mags, and have put A LOT of rounds through them across 6 different rifles), those problems were fixed by 2016. Not a huge sample size, but not small, either, for what it's worth.