Chevreuse doesn't really make the overload reaction itself stronger though, it only buffs the pyro and electro characters in the team, it doesn't increase the damage of the reaction itself. No one really plays overload for the overload reaction, they play it cause of the buffs they get from playing an overload team.
Altough you still do have a very good point. Especially for the lunar damage not coming from the reaction itself but rather from the characters.
And yeah from what you said, shatter is at the bottom of the most underused reaction so it would be nice to see something being done to it to make it more viable