There are few builds where it can be useful, even Dex focused builds. A Ranger with a 20 Dex and no shield has a 17 AC in Studded Leather armor. If they wear Half Plate, they have a 17 AC and a stealth penalty, but if they take Medium Armor Master, they have an 18 AC. So it does only function as +1 AC for them, which they would have gained from Dual Wielder. Thus, it's not really worth it in this case, unless the player really, really, REALLY wants to stack AC.
Unless you're rolling for stats, I have a hard time imagining too many other characters who have medium armor pushing for a 16 Dex, where the choice is between 16 AC with studded (and +1 to hit and damage with dex attacks, and +1 Dex saves, and +1 dex checks), or 17 AC with half-plate and medium armor mastery.
It's worth half a feat at best. The "no stealth penalty" isn't a bonus when you are comparing the choice between studded leather and half-plate, or when comparing the choice between breastplate and half-plate. Going breastplate to half-plate, with a Dex of 16, the feat provides +2 AC. Having a Dex 16 is so marginal, though, that the feat needs a boost. +1 Dex would be fair, I think, without changing much. I'm not too worried about rogues pushing for AC with a shield and medium armor mastery as I think they give up way too much offense not going TWFing.