In Doucoure's case it'll be in part the fee and the fact that many will have taken positions against the signing in the past and would rather stay quiet for that reason, which is fair enough. He wasn't exactly universally welcomed.
To be fair, the standards for being universally welcomed around here are kind of high. He wasn't the best player in a World Cup (and head and shoulders above Messi, who obviously agreed that he stole the Golden Ball from his attitude receiving it). He also wasn't obviously a world-class glue guy like Barry and Milner, the way Allan is.
Off the top of my head, these sorts of things were said at purchase about players that have been successful:
Richarlison - too expensive, overrated home cooking, another McCarthy signing
Digne - who is this rando we're panic buying from Barca?
Godfrey - relegated, buying the best players from relegated sides eventually gets you relegated
Mina - sure, he scores, but Barca wants rid because he can't defend
I mean, we could buy Abraham for 35 million pounds and 15% of the members would probably slate the deal.