Yeah. Performance quality and charisma are mutually exclusive.
I think Cumby gave a fantastic performance, but his Khan was no where near as charismatic as Monty's
He gave a great performance as somebody named "John Harrison" as far as that went. But as Khan, not so much. Even if the characters in this timeline know squat about who he is, the audience sure does, and I hoped to see some elements of Khan from TWOK presented in a new way. Why carryover the name if it adds nothing to the story?
Oh, and I still don't understand when Khan and the Admiral fell out.
Probably when he asked for his crew to be relived - or was caught trying to do so...