Groves might not be a one punch KO kind of guy but his shots are heavy and hurtful. Also he used to walk his opponents into big counters more when he was with the Dark Lord and that made him more dangerous, whereas he doesn't do that as much these days, although he still does sometimes.
Groves doesn't commit to punches like he used to when he was younger. Watch some of his older highlights and you can literally see how heavy his hands are. He used to put a lot of brute force behind his shots. No doubt about it.