I think a good boxer can expose GGG's defense a bit more...but I do think it is by design. Vs. Adama I thought he executed it fairly well, it just looks bad. He lets Adama land the small stuff and GGG slipped all the punches with any meaning.
The concern is that a fighter with quickness could land something GGG won't slip, or a fighter who outboxes him will land the smaller stuff, but force GGG to miss and pile up rounds.
GGG's overall power with all his punches is tough and makes him dangerous. Still don't think there is a fighter currently at 160 or below that beats him. It gets much more interesting as GGG goes up in weight.