It's not so much being down on him as much as trying to properly evaluate him. I'd like to keep him, but probably not for the amount he's going to realistically demand.
I simply think he gives up far too many baserunners and relies on external factors to be a true #2, also taking into account that #2 starters on competitive teams have increased in quality in recent years. They don't have to be aces necessarily, but they're better at preventing runners from reaching base and also have a higher than league average strikeout rate.
Let's take a look at Stroman's 2016 and 2017:
2016: 9.2 H/9, 0.9 HR/9, 2.4 BB/9, 7.3 K/9, 1.289 WHIP, 4.37 ERA
2017: 9.0 H/9, 0.9 HR/9, 2.8 BB/9, 7.3 K/9, 1.308 WHIP, 3.09 ERA
Fairly similar peripherals both years for Stroman, and yet dramatically different results. The main difference between both years? 68.6% LOB% in 2016, and 78.1% LOB%. The fact that his results vary greatly depending on LOB%, which also seems to have variation year to year, doesn't necessarily scream #2 to me.