Bro, we have fans here who write off players for us before they have even kicked a ball for us.
There is an arrogance when people point blank shit all over signings or potential signings because they have seen them play for team x or y under different managers and systems, in different countries/leagues and they don't rate them.
I get that some folks get to watch lots of football and have an opinion about said players but every single player should be given a chance to prove themselves for us, under our manager, in our system, in our league.
Even then, as you have said, they need to be given time to bed in, learn the system, gain confidence.