For an upcoming game, I'm weighing Shield Master as a feat selection. The closer I look at it (and recent rulings on it), however, the more lackluster I find it in comparison to other choices like Great Weapon Master, Polearm Master, Sharpshooter, and even Dual Wielder.
Shield Master's third benefit, the Evasion-like ability to avoid damage on a successful DEX save, is flavorful and useful. It embodies a classic narrative trope of shield-use, and it's likely to save significant HP that would otherwise be lost to breath weapons, fireball and similar spells, etc.
The feat's two other benefits are more dubious.
The first benefit, granting a bonus-action shove if one takes the Attack action, has been the subject of rule-reversals restricting its application to after all of one's attacks are completed. I find that restriction dissatisfying both mechanically and narratively. Mechanically, it means a character using the bonus-action shove will rarely benefit from doing so, because her target can simply move when its turn comes around, before she has a chance to act again. And narratively, forcing a melee combatant into an "attack-shove" (or, for those with Extra Attack, potentially "attack-attack-attack-attack-shove") routine feels arbitrary and unnatural. Imagine a tale of heroic adventure in which a character described as "master" of shield technique did not, could not, ever lead with her shield. For that matter, why is it so difficult, even for a character with Shield Master, to effectively use a shield-bash as a damage-dealing option in melee? Overall, compared to the highly effective and directly beneficial bonus-action attacks granted by Great Weapon Master and Polearm Master, this part of Shield Master is underwhelming.
The second benefit, adding the shield's defensive bonus to DEX saves against single-target effects, sounds useful in theory but turns out to be disappointingly narrow and situational in application. It's not unrealistic to think a character with Shield Master could make it through an entire campaign and never use this benefit. It's hardly a benefit at all.
Therefore, I'm considering a house rule modifying Shield Master, by adding the following two bullets:
You are proficient with shields as improvised weapons.
When you are wielding a shield as an improvised weapon, you can use two-weapon fighting even when the one-handed melee weapon you are wielding in your other hand isn't light.
This modification seeks to put Shield Master roughly on par with Polearm Master and Dual Wielder. A character with Dual Wielder can wield, e.g., two longswords with a d8 damage die, or a d8 longsword and a d4 whip with reach, and still gain a +1 to AC. A character using Polearm Master won't get a shield's AC bonus, and will only have a d4 damage die for her offhand (i.e., "opposite end") attack, but adds her ability modifier to the offhand damage, is likely to enjoy a polearm's d10 damage die for her primary attacks, and has reach on all her attacks. By comparison, with this modification, a Shield Master character can enjoy the +2 bonus to AC from her shield and still utilize two-weapon fighting, albeit with just an unmodified d4 damage die from the shield. Or, thanks to the proficiency piece, she can viably lead with her shield, treating the shield as her primary weapon and using her normal weapon for a bonus-action attack with unmodified damage. She could even lead with a shove using her Attack action and still get a bonus-action attack via two-weapon fighting -- just as she could with Polearm Master or Dual Wielder.
Nevertheless, it's possible I'm missing something. Are there foreseeable balance problems here? What are they?