They are super fun. But get beat BADLY by Fog Cloud.

Essentially their eye rays are targeted spells. If they have no target, the rays can't fire.

Definitely a design oversight.

Now that's technically the rules. If you want to house rule something about the Beholders being able to target spaces while making sure the players have advantage on the save that would more than likely be acceptable.

But just be aware that RAW the beholder rays can't target what it can't see.