Gerrit Cole hasn't been a top-end starter the past two years, missing time with elbow and triceps injuries in 2016, pitching a full season in 2017 but with worse results than he had in his breakout 2015 season, when he was worth 4.5 WAR by his ERA and 5.5 by his peripherals. The Astros didn't exactly have a hole in their rotation, but even if Cole repeats his 2017 performance he's a soft upgrade, and the package Pittsburgh receives is more one of quantity than of impact.
Cole gives up too much hard contact for a pitcher with his caliber of stuff, and became particularly homer-prone last season; he gave up as many homers just on fastballs (18) in 2017 as he did in his previous two seasons on all pitch types. Cole has a tendency to try to throw his way out of trouble, which causes his stuff to flatten out. His four-seamer has always been true, but he can show a plus slider and changeup when he doesn't overthrow them, although he's still vulnerable to quality contact when working in the strike zone, especially in the middle third of the zone if you divide it vertically. He can get away with four-seamers at or above the top of the zone or by burying a slider at the bottom; in 2017 he missed those spots far too often.