Reaper is garbo-tier. He gets shut down far too easily and very simple to spot when he's getting ready for ult. Time has passed and you can't see enemy Reaper fighting? He's either above or lurking somewhere, waiting to ult.
As Soldier, I just run around looking for him hiding knowing he has ult and force him to flee or kill him if he's unaware. My teams usually feature a Hog, so he knows to get his hook ready for the inevitable ult.
This season, I think I have only ever seen him in KOTH and can count on one hand the number of times I've seen a multikill ult. I'd run out of hands counting the amount I've seen someone switch from a Reaper though. In his current state, he's just not that good.