Pitchers are different from hitters, rarely do you see a hitter come out of nowhere, they may make incremental improvements, get stronger, more disciplined, etc. There are examples of hitters fixing a flaw in their swing, but it's rare to see them totally outperform their past (they may outperform an off season or two).
Pitchers tend to be more erratic because for a young pitcher, there are more "moving parts." Especially for bigger bodies, it takes time to smooth out the mechanics, find a delivery slot that works with your pitches, then learn to repeat that motion 100 times a game without giving away your pitches.
Thompson was solid, Eflin was lights out in AAA. They were both 22, young for the league, they both struggled in the Majors but showed extended flashes of what they could do in a "groove." So do you project off their history or a few games when they got knocked around in the majors? Eickhoff didn't make major changes, he improved his curve, improved his command a little, starting throwing a 2 seamer more and eventually got more movement, his slider is still inconsistent, his changeup a work in progress. That's why it's hard to project pitchers, even little improvements can have a big impact.
Velasquez is a good example, he had a great start, then struggled the rest of the year. No matter how good his raw stuff may be, if he can't develop the command to go more than 5 inninngs, he's never going to be more than a 4th starter - and that's why pitching is just harder to project than hitting. But those kind of improvements are much bigger judgment calls than say his velocity or movement of his pitches.