Well consider this angle: cool unique racial units.
You know what made Dark Elves cool and fun to play? Spider Queens, Shades, Incarnates.
If Dark Elves are just a skin laid over other units? That’s pretty lame.
This extrapolates to every race in Shadow Magic. Everyone had cool, unique units and stats. You could tweak those units with your own flavor via magic and the like, but Shadow Demons were nothing like Halflings at the end of the day who were nothing like Tigrans.
Now arguably AoW3 already destroyed these cool units anyway, but I’d take the position that it’s a negative. I enjoyed AoW3 a fair amount, but nothing like I enjoyed Shadow Magic. At the end of the day Race Riding Racial Mount is lame compared to Sphynxes. Fairy Dragons and Gluttons. Class-based Race Unit is also not that exciting, though at least the uniqueness of each race came through a bit. Your Orc Crusaders felt different than Goblin Crusaders for the most part.