When did you see him? He's been improving a lot over the last 6 months, going on an Authom-like climb back up the rankings. The main difference between Maxime and Niels is that the latter has been there (in the top 200) before, and should be able to do some damage over the next couple of years. I'm very happy with both right now though, providing good support for Belgian tennis.
I saw him at a Future and a Challenger in the Netherlands. Not recently though. I know Niels was a good junior, but he has really not managed to deliver on the professional tour. I like his game more than Authom's (one of my least favorite players), but he strikes me as one of those successful juniors whose game does not translate well onto the ATP Tour. But it might still come together! So many players are breaking through at around his age.