Dwight Howard is still a monster and has probably become underrated lately. I, too, would be frustrated if I were him especially with James Harden and that goes for both ends of the floor.
On defense, the case is more obvious. Harden is one of the worst defenders in the entire NBA. For a defensive stalwart like DH12, that must be very frustrating to play with a guy in Harden who's defensive play can really only be characterized as lazy.
On offense, Harden is a truly elite and special player. But DH12 is pretty good himself. He's shooting 60% from the floor this season, after shooting 59% for the last two seasons, and his field goal attempts and usage rate are plummeting. This season, his fga/36 min (just 9fga per 36 min) and his usage rate is below teammates like Terrence Jones, Corey Brewer, and Marcus Thornton. My one critique would be that his turnover rate is too high (like it's always been and even a bit higher this season), but when you're shooting 60% from the floor and hitting ft's at at least an acceptable rate then yeah, maybe you deserve the ball more.
Now the question turns to fit. Does he fit with the celtics? I'm not sure. He's still a really dominant defender (not as much as he used to be or any longer top 3 guy, but probably at least as much or more than he's been given recent credit) and he's at least as good offensively as what we've already got at the position. He costs a lot of money and he's not in our team's age range really. I'm very on the fence. If we don't have to gut our rebuilding process to get him, I'd be ok with it. Lee + Sullinger + Zeller + the dallas pick + and our 2016 pick? Sure. But is Houston interested in that? They may be interested in sliding Capela into a starting C role. I'm not really sure what they'd be looking for in return.