I think he's a brilliant coach, his style of play has been praised, not just at Brighton but at Sassuolo and Benevento before that as well (you can't really rate his Shaktar career because of the war). He took Sassuolo to 8th two seasons on the trot and just missed out on European football to Roma who were ahead on goal difference.
He likes to dominate possession as we've seen at Brighton, even at Sassuolo his team recorded the highest possession share in Serie A. He stretches play relying on full backs and wingers, bringing the goal keeper into a back three while passing out from the back to give his team an extra man. The downside is his teams can concede too many chances and if they're not on top form it can go wrong.
Players seem to love playing for him and certainly develop under his leadership. Dunk said quite early on in RTDZ's time at Brighton that De Zerbi made him see football in a completely different way. In fact I would say the majority of Brighton's squad have played better under De Zerbi than under Potter and I can not think of a player that's got worse.
A lot is made of the injuries he's faced and Mac Allister and Caicedo leaving. But our drop of in form in the second half of the season possibly wouldn't have happened if we'd used the January transfer window more. RDZ wanted to bring in a couple of established players to bolster the squad, but our board refused and only bought in a 19 year old left back, Valentin Barco, typically of Brighton, one for the future. That was really the biggest issue for RDZ and that's when he started questioning the club's ambition vs his.
He is definitely passionate and determined, he develops and improves players, his football is possession based, attacking and exciting to watch using the wings, he's great with young players as well as established ones. I'm gutted, but not at all surprised to see him leave Brighton. He's been highly rated everywhere he has been, even when he was relegated with Benevento, years ago now, the Italian press raved about his innovative and exciting style. I don't get what's not to like about him, I think he'd be a great fit at United if you decide to move on from ETH.