Ancalagon
Dusty Dragon
The specter is what you are forgetting (that and the significantly increased critical hits) - it's about the best combat pet available to any class. It's resistant to most damage, giving it respectable survivability, it's attacks do good damage (3d6) and reduce maximum hp. And it can walk through walls. And it doesn't use the warlock's action economy to attack. Give your single class hexblade a flat +10 DPR when trying to whiteroom it.
Good to know.
Multiclassing does delay the acquisition of higher level features (a mutliclass hexblade definitely takes longer to "come online" than single class). Is the wait for the specter enough to fully make up for the benefit of multi-classing?
As far as the critical hits, that is available very quickly to the multi-class character, so I am not sure it "counts".