I have run a couple of combats now against antagonists that are able to charm PCs. I like the flavor of the ability, but I feel like it runs a severe risk of being unfun by virtue of taking one or more PCs out of the fight completely, leaving the players to sit out in boredom. I'm thinking of things like the Dryad's fey charm or the Vampire's charm ability. Once the charm takes effect, the target doesn't get any more saving throws; the only way to even get a chance of breaking the charm is for the monster or its allies to harm the PC.
Now, a vampire might want to bite the PC, but a Dryad has no reason to harm a target she's already charmed, right?
How do you handle monsters with charm abilities? Do you typically find a way for the monster or its allies to harm the charmed PC?
Do your players enjoy or resent being charmed?
Do you typically grant extra saving throws? Allow other party members to snap the charmed PC out of it?
If you run it by the book, do you find that this leads to combats ending in a TPC (Total Party Charm)?
I don't have experience running 5e vampires in combat yet, but in regard to the dryad's Fey Charm, there are more ways to be freed of it than you're representing, and total party charm isn't possible due to the 1 humanoid limitation. Here are the relevant paragraphs from Fey Charm:
Each time the dryad or its allies do anything harmful to the target, it can repeat the saving throw, ending the effect on itself on a success. Otherwise, the effect lasts 24 hours or until the dryad dies, is on a different plane of existence from the target, or ends the effect as a bonus action.
If a target 's saving throw is successful, the target is immune to the dryad's Fey Charm for the next 24 hours. The dryad can have no more than one humanoid and up to three beasts charmed at a time.
"Harm" might cover more ground than physically roughing up a target. Stealing from the target might constitute harm, for instance if the target has a beautiful necklace that the jealous dryad wants, but the loss of that necklace would ruin a critical quest – as DM I could see that being construed as harm.
After a day, the target can attempt its save again. So, if a party is willing to parlay and not resort to violence, they could buff up the target's save in various ways (e.g. diplomatically arranging for the paladin to be within 10 feet, the Bard singing a song to the dryad disguising Bardic Inspiration to his charmed ally, guidance, and so forth).
Killing the dryad ends the charm.
It's niche, but transporting the target or the dryad to a different plane ends the effect. For instance, if there's a passageway to the Feywild in an oak nearby, the PCs could cast compulsion to move the dryad within, trick/coerce her within, knock out the dryad and toss her within, abduct their companion and flee within.
Parlaying with the dryad is possible, for if they can convince her that the charmed PC has an essential quest to complete which benefits the dryad or nature in some way, she could release the PC as a bonus action.
Finally, a clever party could trick the dryad into charming someone else (probably a NPC), which would cause the charm effect on the previously charmed PC to end.
It's unlikely all of these would apply in every situation, but I could see at least two or three of them applying in most.