Just started this game, and I'm playing Free Folk, so can't run neutral units. I've heard that Walder Frey and Varys are very commonly run as the NCUs of choice, since Walder Frey can shut down units' abilities (it's pretty shitty what he does to giants) and Varys can cancel Walder Frey. If this is true, do you think this is healthy for the game? Should an ability like Frey's exist in the game?
I think that Walder Frey was a hasty fix to "super units" with particularly effective combinations of attachment abilities and/or unit abilities. On paper, I think his ability is interesting and worth considering as a mechanic of an NCU but the problem arises in a number of ways that I think are problematic to the game's health as a whole:
At any given time, a player has (assuming their army is full strength) anywhere between 4 and 10-11 combat drops on the board. Of those, you might have to worry about muting 1-2 of them; Free Folk Raiders, after all, are not worth muting but Giants are. Guardsmen the same story as Raiders, but Pyromancers the same as Giants. On the lower end of combat drops, this means that Walder can not only cripple 1/4 of your board presence but likely can mute the only important unit or one of two important units on the table at that given moment (say, those Tully Cavaliers in prime position to smash your flank or those Dothraki Vets set up to clear a unit). For 5 points, this ability is bonkers and only becomes more bonkers the more elite the army that gets Frey'd is.
Frey's status as an NCU makes him rather tricky to deal with as well. Short of the Stark version of Jacqen (who requires not only that you take Arya and a 2 point investment, but that you also destroy the unit he's in which will take at least 2-3 rounds), there are no real ways to remove Frey permanently. Though there are objectives that can mute an NCU, these are not always guaranteed in game modes with random missions and will require a shift in your battle plan if such an objective is present. You still get points for holding the objective, but your battle plan has to shift to sufficiently guarantee access to this objective which might mean that Frey is indirectly paying for himself anyways just by virtue of being in your opponent's list.
The counterplay on the NCU board for Frey is also very limited. Though Frey's ability will only trigger innately for 3 rounds without holding the Crown space, even if the opponent of a Frey-using opponent goes first they have to make the call of claiming the Crown (which is not beneficial or an efficient use of NCUs for several factions but is largely always beneficial for a player with Frey) or make a riskier or inefficient play with the unit that is obviously going to be Frey'd in the following turn. This problem is only made worse if the player with Frey has forced a difficult situation upon the player - this is not to say that good maneuvering and tactics should not be rewarded by making your opponent choose between suboptimal choices, but rather that Frey makes it vastly easier just by virtue of Frey being in the list.
To summarize - I think that Frey, even if he is not necessarily a no-brainer and has not made his way into every list, has problematic impacts on the game. He is a (virtually) 0-risk piece that, even if he does not utilize his ability once, requires an excessive amount of maneuvering and sacrifice on the part of the non-Frey-owning player such that he will always pay for himself. Frey himself is not an example of playing the game well, he just makes winning the game easier at the cost of 5 points (only 2 of which are really going to his ability).
In re-balancing Frey, I would shift around his ability to either function on an order system that replaces the zone's effect he lands on with his muting ability (say, 3 times a game) and keep that his influence always deals a wound. That, or make it so that the muted unit does not lose its abilities for the whole round and instead loses them until the end Frey player's next turn (allowing you to buy yourself time to maneuver into place to counter a threat, or temporarily make a unit easier to deal with without utterly neutering the unit and giving the timing of Frey's placement some actual thought process involved).