We'll sign superstars that turn out to be duds, we'll sign some dross players that turn out to be decent for us. It happens in football, not just for us but for everyone.
There is only merit in evaluating players based on their performances in an NUFC shirt, because what they've done with other clubs before us or after us is pretty irrelevant.
At this moment in time, we have a world class manager choosing the players rather than a fraud like Pardew, I'll more than happy give any person Rafa chooses to sign a decent chance. He's bound to pick some duds, but every manager does. If Rafa thinks that Joselu can do a job for us, then I'm more than happy to trust that judgement until I've seen otherwise. Clark and Murphy, two signings last season that 90% of our support, me included, thought were shit - but two players who ultimately helped us win the league and promotion at the first attempt.
That said, I understand why people would like to evaluate players before they sign - although I just don't think there is much to it and there is absolute no point in writing someone off as 'shit' before they've even kicked a ball for us.