Downsides to letting a Fighter have a shield and two-handed weapon out in the same turn?

Question

I've just started DMing a D&D 5e game and we're playing fairly fast and loose with the rules since my party's in it for the roleplaying more than the specifics.

One of my players is a half-orc fighter wielding a two-handed warhammer and a shield. She asked if she could (in a single round) stow her shield, take a strike with her warhammer (with both hands), then get her shield out again. From what I can tell in RAW this wouldn't be allowed since it would take essentially three actions - one to switch to the warhammer, one to attack, and one to switch back to the shield, but I allowed it at the time since I didn't see any reason not to as a house rule. We've been treating switching weapons as a bonus action anyway so in our eyes it's just an extra bonus action per turn and it does make it impossible for her to do anything else on a turn when she chooses combat.

I don't particularly care if I'm right or wrong on the rules here (since we're fine with house rules) but I am wondering if allowing this will cause me trouble later on. Are there downsides I haven't predicted to her having her shield always accessible in every round of combat going forward? It bumps her AC but as the lone tank in a 2-player L4 party they're fairly underpowered in general and this seemed like a decent balance, but that may be my naivete talking.

The biggest "issue" I predicted was just that it would give her basically a permanent bump to her AC. As addressed though, since she's the sole tank and I've still managed to hit her at least a few times in most encounters I don't see this as something that can't be dealt with by boosting monsters a little to match, if needed.

I'm mostly concerned this might affect things in later levels that I haven't researched yet; this is my first time playing D&D myself rather than watching others and I'm far from fluent.

Answer 1

Ultimately, this is all your call as DM and how/if you want to accommodate your campaign development to make any changes.

From a rules perspective, players get a free action (PHB pp190) that allows them to do things like draw/sheathe weapon, pick up a dropped item, etc.

Shields further require an action to don/doff (PHB pp146). Given that they add +2 to AC, that is a big boost and possibly why they require the action in order to gain such a bonus.

The use of two-handed weapons can also be seen as a balance issue. You are choosing bigger attacks over more defense. If you opt to let someone with a two handed weapon also get the benefit of the shield, you are greatly increasing the capabilities of that character.

There is a downside to allowing them to switch between their 2-handed weapon and shield at will: any out-of-turn actions (like Opportunity Attack reactions) will be without their primary weapon. They'll be using unarmed strikes or improvised weapon which would deal either only 1d4 if using the shield to attack (no proficiency bonus), or 1 damage if using unarmed strike.

Letting them don/doff the shield without penalty during combat basically gives them an extra 2 actions if you go purely by the rules AND they still get to get the damage benefits of the two-handed weapon. If they also have the Great Weapon fighting style and/or feat, then the balance becomes even worse.

Many of the combat/feat/fighting styles lean towards either being a DPR tank or a damage soaking tank. Letting someone get both could unbalance, but you as the DM are generating the combat scenarios and can accommodate for anything you give (or don't give) your players.

I also wouldn't get too hung about that character "being the only tank." Having a tank isn't required, just like having a healer isn't required.

Potential Solutions

Provide a means of access to magic protection (better armor, magical armor, ring/cloak of protection).

Give them access to Shield of Faith (via feat Magic Initiate), but still maintain the concentration mechanic. This will give them the +2 they're looking for, but at a risk.

Answer 2

It is fine, and will be fine

You did not write what kind of Fighting Style she uses, so let's evaluate all.

Dueling

During her turn she does 1d10+3 instead of 1d8+2+3, and she doesn't benefit from a shield. Strictly weaker than without the houserule.

Great Weapon Fighting

During her turn she does 9.3 damage compared to 11.33 that she would do with a Greatsword. Out of her turn she does 7.5 damage instead. In return she gets +2 AC. Not bad, but also not remarkable. However, she also has to pay a bonus action. Rather weak altogether.

Great Weapon Master
She pays an ASI like everyone else, but she gets nothing out of it, as the bonus action is already spent, she can't attack again on a kill or critical. Her damage is also still more than two points behind a Greatsword.
The Warhammer is not heavy so the -5/+10 doesn't work either.

Any other Fighting Style

Compared to a usual Warhammer and shield fighter, she does +1 one damage on average (1d10 instead of 1d8), in excange for this she has -2 AC during her turn. I would say this is more or less balanced.
In addition, she has to spend her bonus action on all this shield moving. For me this is quite a high cost, she can not use the Shield Master feat for example. Everything considered, I would say she is below average.

But she can use a bonus action for something that would cost two actions for others!

This doesn't matter, the original cost is completely irrelevant.
Imagine an errata comes out, and doning a shield is now one full minute. Did the half-orc get any stronger? Not a bit.

So we only calculate the current cost against the current benefit:
She gets a shield for a bonus action, is it overpowered in combination with all the damage cost she pays?
Very far from it.

Using a Greatsword with a shield would be overpowered, but she can't do that.