“Best map for Heroes of Might and Magic III” (2007).

Maybe it's simpler: if the map is enjoyable to play, then it's playable; if you don't enjoy playing your own map, then it's not playable.

I agree, that's pretty much how it is.

P.S. But in my understanding, "playability" is like in chess – not every game is the same. In terms of Heroes, "playability" means non-linearity. That is, the map should offer several different scenarios for completing it, which would differ from other scenarios (in subsequent playthroughs on that map). The more such scenarios there are on the map, the more "playable" the map is.