My only experience is with the 12 rd Glock 26 magazines. They are a bitch to load to capacity when brand new. I had to dig out one of the Glock magazine loading tools to get the 12th round in to one of the magazines. On a hunch, I fully loaded both of my 12rd MagPUl magazines and let them sit for about 10 days. They're still a bitch to load to capacity, but now I can get the 12th round in without assistance.
I only have about 300 rounds through my two magazines. All of which is 115gr WWB or Federal American Eagle and about 50 ads or 147 gr Winchester Ranger T series. No issues at all. I should probably vet them a bit more, but I do use these magazines on the rare occasion that I bench the Glock 19 and carry my G26. I also own a Glock OEM 12 rd magazine. I greatly prefer the OEM magazine as far as fully seating a fully loaded magazine into the gun. It just, "clicks in" without effort. But I have tried to limp wrist the gun to induce a malfunction when using the MagPul magazines and they have been reliable to date.
I do not shoot steel case ammunition so I an not comment. Our duty round is the Ranger T series, so I can't comment on the HST or how it feeds.
The reason I prefer the MagPul 12 mag over the OEM mag is the feel of the mag when loaded. The OEM base plate on the 2 rd extension sits rearward a bit. THE MagPul baseplate feels more natural to my pinky and overall just feels more natural to my hand.