IMO they're best as a supporting monster adding chaos to a highlevel fight, rather than a main enemy. 5e beholders are very random and can get very hard countered (or even just shot to death pretty quickly. They are not tanky) so they're not great as a boss monster because its just too whacky and wild.
But if they're just there to be a supplement monster to a superpowered boss? The chaos is great for spicing things up.