Is it playable alone?

It's definitely playable in single player. Most of my hours are in single player, which I suspect is true for many people. Zombie hordes - whether Blood Moon hordes, wandering hordes, or sleeper zombies in a POI - scale up or down based on the combined player level of the player(s) in proximity. So when you're just one player, the threat stays manageable.

There are definite advantages to playing as a group, though. You can branch out and have each player specialize in different skills. And when fighting, it's handy tactically to have zombies splitting their attention between multiple targets.