A decent chunk of the best WR’s in the league in recent years were 5th year seniors. It’s a nonstarter of an argument. Either the guy is a good prospect or he’s not. The 5th year aspect really has no relevance.
The VAST majority were not. Being a 5th year senior gives you a ridiculous advantage as a collegiate athlete. Does it mean they won't pan out? No. Nobody is saying that.
Just that it adds a degree of difficulty to their scouting.
A great example of this is Xavier Legette out of SC last year. He looked like a man amongst boys in college and not nearly as impressive in the NFL.