You got me to dig around on the internet, and I won't pretend I'm some kind of an expert, but here's what I gather from various comments by fans and articles covering his play.
He was a Bundesliga rookie of the year and by all accounts was highly regarded, and fans felt that he had a lot of potential but fell off before the transfer to Napoli. They liked him and hoped he would find his old form after the move to Napoli, but he didn't see many minutes there and then picked up some small injuries. He was mostly used as a late substitute all over the pitch, but mostly on the left.
The consensus from article writers seems to be that he failed to catch on in Serie A because they played him as a winger rather than centrally, arguing that he had most of his success in the Bundesliga as a #10, but this seems to be completely off. All heatmaps of his actual play show that he mostly operated as a right winger in the Bundesliga. When you look at sites like fbref, he gets labeled an attacking midfielder, but I think that's inaccurate. Oliver Glasner's favored formation seemed to be a 3-4-2-1 that became 3-4-3 in attack.
If anything, he seems a right winger who didn't catch on in Napoli and for whatever reason played out of position on the left and elsewhere.
Conclusion? What do I know?