I said this about rath, but it could be applied to miko.
There needs to be an easy support character due to how the game is set up, I don’t think it detracts from the game, and characters play differently enough that it isn’t an issue.
If there was another character with a self heal and a beam heal who could stick to people, but required extra effort, even if it was for extra reward, then this would be a problem. But there isn’t. Miko is a healer character for people who like that style, and perhaps don’t want to deal with the complexities of something like kleese’s playstyle.
Perhaps miko should be easier to kill, I don’t know. Perhaps he shouldn’t be able to heal himself as strongly, I don’t know. But him being an easy hero to play is not a problem in and of itself.