The limitations seem to stem mostly from the view about Kupp not being fast enough in his
timed speeds. The perspective of him only functioning as a slot WR are related to this as well. There is some logic to support this as NFL corners are very fast, so matching up on the outside becomes more difficult against these players for a slower WR. There is also some charting data on Kupp that supports the view of him running more routes from the slot position than from the outside. He did.
What gets lost in this evaluation however is some of the other metrics on Kupp are very good, for example his 3 cone times 6.75 combine and 6.53 at pro day are excellent and you see him demonstrating that change of direction ability on the field when he breaks off his routes, there is some quick twitch ability being demonstrated and that is why he gets so open because of sudden he is with his breaks. This is something that translates into successful WR play from any starting point of the alignment. He is not limited to just running drag routes across the middle and looking for a match up against slower players because of his route running ability and being able to create separation with sudden breaks of his routes.
He reminds me a bit of Jordy Nelson as a route runner and I can see him winning in similar ways as Nelson in the NFL.
Another player who people have pegged as a slot receiver only is Stefon Diggs, but what Diggs does to win works from the outside as well. Kupp is a taller player than Diggs, but I see some similarities between them as far as how they break off their routes quickly and leave defenders in the dust because of it.