D&D Beyond shared some stats about the things people are using from Explorers Guide to Wildemount. These are stats from 28 million characters.
This is a fun little idea, but it is not overpowered.
A battlemaster will, generally, do more damage. They also do not have a risk of having their entire schtick negated by a damage aura or a readied attack from an underling.Just the ability to have your actual character stay behind cover while your infinitely re-summonable dupe makes ranged attacks and draws enemy fire can get ridiculous in many dungeon situations. My player has so far spent two sessions mostly doing this. Particularly devastating against enemies who aren’t overly bright.
A battlemaster will, generally, do more damage. They also do not have a risk of having their entire schtick negated by a damage aura or a readied attack from an underling.
Solid, but not overpowered.
I mean, it totally is. All three subclasses in Wildemount are.
I love the concept to pieces though. It was also really funny to me because I was playing a totally bizarre character in Path of Exile, and thought "D&D could never do this", then the Echo Knight came out and is basically a very similar concept.
I agree with your assertion re: these likely being "test characters".
All three Subclasses were put through the standard private playtest wringer, so I would bet they are in fact balanced, outside of white room scenarios.