Does adding "+1 Str" to the "Medium Armor Master" feat break the balance?

Question

There are 5 "armor" feats in PHB: Lightly/Moderately/Heavily Armored, Medium Armor Master, and Heavy Armor Master. Four of them add +1 Str, of those some allow +1 Dex instead; Medium Armor Master adds neither. I am considering that I let my player take the Medium Armor Master feat with +1 Str. Is that balanced?

I think it is, and my reasoning is as follows: Medium Armor Master adds two bonuses:

1. no disadvantage to Stealth checks due to medium armor (some medium armor types impose that);
2. Dexterity bonus to AC is capped at +3 instead of at +2.

Compare this to the Heavy Armor Master feat, which grants +1 Str and -3 reduction to received bludgeoning/slashing/piercing damage. -3 damage reduction is good but scales worse than +1 AC - that might explain why there is no +1 Str for Medium A.M. On the other hand, while that -3 damage reduction is immediately applicable to anyone, +1 AC requires and Stealth check assumes high Dex and +1 Str is of real use only to a Str-based fighter. So anyone wanting to use the modified Medium Armor Master feat to its full potential need to invest both into Str and Dex, which obviously costs other opportunities.

Is there a flaw in my reasoning and the modification indeed causes a disbalance?

The actual situation is that a Str-based Ranger in my group often serves as the group tank, and the initial build gave her Str 15; so the modification is a bit tailored for her. Other players will likely be fine with that, but that should not make the question irrelevant.

Answer 1

This benefit is extremely niche

You have to have high Strength, high Dexterity, and care about Stealth checks for this change to matter.

• If Strength < 14, you should never attack with Strength and thus don’t really care about the Strength bonus.
• If Strength ≥ 14 and
  • Dexterity < 16, Heavily Armored provides superior AC.
  • Dexterity ≥ 16, Heavily Armored provides the same AC that Medium Armor Master does, and also provides the same +1 Strength as your change.

Realistically, you should not be attacking with Strength if your Dexterity is better, so really to care about the +1 Strength, you need Strength that is as good as, or better than, your 16+ Dexterity (after the bonus).

Having 15+ Strength and 16+ Dexterity is very unlikely, and unless you rolled those and a 16+ in Constitution, I’d argue you’ve made a mistake. Even if you do have all of that, Heavily Armored is identical in every way except for Stealth.¹

So all you’ve really done is made high-Strength, high-Dexterity characters better at Stealth. That doesn’t change the fact that such characters are extremely rare (and not well-rewarded by the system). They either rolled preposterously well and this is just a bit of win-more for them, or they have paid far too much to get to this position and this isn’t going to make up for that. Neither is going to change much about the qualitative experience of your game.

In short, it won’t imbalance anything, but it won’t fix any balance issues either. The characters who benefit either don’t need any help, or need more help than this change is going to give them.

Since this ranger is at Str 15, they are in the one of the best candidates for the feat—if they have Dex 16-17. If they have Dex 18+, they should be attacking with Dexterity, not Strength, and so the bonus won’t matter much at all. If they have less than Dex 16, they should be taking Heavily Armored. But assuming they fall in that sweet spot, it’s a nice bonus for them—and unless they rolled those scores and a 16+ Constitution, they probably need it.

1. _{Heavy armors cost and weigh more, so that is different—if your group is tracking encumbrance or starkly limiting access to gold pieces. But the default game doesn’t worry much about such things. And even if you are tracking them, at Str 14 you have “enough” Strength to carry your gear, and the 750-gp difference between half plate and full plate isn’t that big a deal. Neither concern merits much consideration stacked up against the opportunity cost that is a feat. Athletics checks, and the rare Strength saving throw, are in a similar boat.}

Answer 2

The feat is not underpowered

If we assume that the character does not have access to magical armor the feat effectively allows the character to gain a +2 to AC while not having to deal with disadvantages on stealth.

• The first bonus of the feat, removing the disadvantage on stealth checks, allows them to freely consider different equipment for their build. They are able to use half-plate armor without penalization on their stealth. This increases the base AC accessible from 14 (breastplate) to 15.
• Increasing the limit on the maximum DEX bonus applicable from +2 to +3, they are able to get an additional +1 AC

If they are able to take advantage of both of these bonuses, the benefit is certainly worth a feat when compared to other feat options in the PHB.

Feats that increase stats in addition to giving other benefits to a player often only give situational benefits. For example, the mentioned heavy armor master feat only limits the damage reduction it provides to nonmagical weapons which makes it significantly weaker for higher-level characters.

Adding a +1 to strength to the feat is not going to break the game

However, it will make the feat a much stronger option as increases in ability scores are powerful upgrades to a character.

What you are suggesting is not really moving a weak feat on par with other stronger options. You would be giving one of the characters in your party a small power boost. If that is something you feel is needed or something the group agrees on then it is fine.

Alternatives

I will not go into details since this is not the primary focus of the question but there are other ways to achieve similar effects in a character build to what medium armor master offers.

• Mithral armor does not impose disadvantage on stealth checks by default, a character with access to that might want to invest their ASI/feat differently than into medium armor master.

• Pass without trace is a very useful ranger spell. It does not help with the stealth disadvantage, but it makes sneaking much easier even for walking clanking tin cans.