Short, he's smart and really talented.
He lacks a good sprint, but it's not like he's really slow either. In his neo-pro year he was 3rd in a bunch sprint once (Handzame Classic if I remember correctly). The problem is that all classic riders have a good sprint nowadays, so in a group sprint there will always be someone faster. Hence why I think he'll never be someone who wins a lot. But when he wins, it will be a big one (like Strade Bianche).
In Belgium there is a debate about what type of rider he is and what type of races he should be focusing on in the future. He's the kind of rider that can do well in literally every classic there is and he also proved that he can climb pretty well for a 23y old. Contrary to popular belief I think he'll be better in the Ardennes classics and San Sebastian, Lombardia, ... than in the cobbles classics. Because of that 5th place in Flanders when he was 20 years old a lot of people in Belgium thought he was the next Tom Boonen. I've never believed that taking into account his results in the u23 category where he was impressive in races as Tour de l'Isard. I don't think he'll ever be one of the best riders on cobbles, but I do think he can be world class in the more hilly classics. Which doesn't mean that I don't think he can win Flanders, he can. Roubaix though... I don't think so. That 5th place made him choose the Flemish races over the Ardennes in his first two years. Last year he finally did Amstel and I'm convinced he was one of the best that day. Without bad luck he would've been in that first group. I'm very excited that he's also doing Liège this year.
Oh, he's also a real "flandrien". Just like Wellens, he loves wind and rain. He doesn't have a problem performing in the heat though.