Here's some excerpts on the reasoning from the scouting report (not the full report):
The only knock on Stroman has been well documented – his height. Standing at a shade under 5’9, he’s not able to naturally create a downward plane that a taller pitcher would. This makes his pitches slightly flat and easier to hit but he does a very good job staying on top of the ball by working down in the zone. His loose arm will also cause him to lose his arm slot at times.
It seems a bit funny to think about now, but less than a year ago many people were still tagging Stroman with the “reliever” tag, which came more from his height than ability. The track record for short starting pitchers isn’t great but Stroman should be able to hold up as a starter for at least a few years, while his stuff is still at it’s best. Once his arm inevitably starts to deteriorate it will be tough but, until that time comes, he’s a potential number two starter. Worst case scenario, he’s probably an elite reliever or back-end starting pitcher.