He is criminally underrated.
Stroman ranked 23rd in all of baseball in fWar in 2014, despite only starting 20 games. He was injured most of 2015 and put up 4 solid starts after most assumed he wouldn't even be able to return. He put up 3.4 fWar in 2016, ranking 19th overall. Followed that up with 3.2 in 2017, placing him 20th. He had a down and injured year in 2018, but if you extrapolate his war based on a full season, he'd still rank around the top 30. And now he's pitching better than ever, and is currently ranked 19th in fWar, placing right behind Soroka and Verlander. He's been extremely consistent. Other than a team like the Stros, Dodgers, and Nats, right now, he'd be the most solid number 2 a team could expect...and a number 1 on many (that's not meaning he's an ace for those who will argue that point).
Add in the fact that he's only 28, has an extra year of control, and the trading team would have an ability to try and extend, anything other than a top propsect and other pieces would be a massive disappointment.