We're looking to use Medium Armor Master to it's full potential here:
- Wearing medium armor doesn't impose disadvantage on your Dexterity (Stealth) checks.
- When you wear medium armor, you can add 3, rather than 2, to your AC if you have a Dexterity of 16 or higher.
Without really trying to push every aspect of this feat, we need to think about what type of characters benefit from this:
- Any character running at least 16 DEX (ideally making STR a dump stat)
- Still wants to be useful in stealth situations (maybe the entire party is stealth and you don't want to be dead weight to them)
The prime example of this is a DEX Fighter that wants to still be sneaky while maximizing their Armor Class (AC). Some more obvious situation I can see for Medium Armor Master is a Bard or Ranger looking for a clean way to increase their AC while still preserving their Stealth skill. There's a chance a Barbarian might pick this up since their class features work with Medium Armor but not Heavy Armor. Hexblade Warlock, Druids, and Artificers might be warm to this feat but the real sleeper is the Cleric.
The usefulness of this feature will be higher at tables that use the point buy system for determining starting ability scores because players will either want to shore up their 13 or completely ignore it.
The biggest hurdle for this feat is that most characters will want to attack with their DEX modifier instead of their STR modifier, so it removes the usefulness for melee builds that focus on Great Weapon Master and/or Polearm Master. Another problem for this feat is that it directly competes with the Defense Fighting Style, so we'll largely need to stack this Fighting Style with Medium Armor Master (otherwise, you're better off wearing heavy armor or taking the Defense Fighting Style instead) -- sure, there might be a situation where someone takes this feature to clear up their Fighting Stance options but I'm working with the assumption they want both.
An Incredibly Niche Situation
Paladins and Fighters only get heavy armor proficiency if you take the class at character level 1
There may be a handful of other problems with this example but a Dexadin (DEX-based Paladin) would have AC equivalent to their more standard Strength-based counterpart. That said, if the same Paladin wanted to multiclass to a Sorcerer for better saving throws or a Dao Genie Warlock (instead of Hexblade), Medium Armor Master is a reasonable PYF (pick your favorite) feat around the middle tiers of the game.