Sancho's ability is so transparent. I said it in his thread, hes a player that depends a lot on the ability of other players because he lacks certain physical attributes. He's good off the ball, he's a good dribbler, very technical and he's tactically aware. Unfortunately he's not that quick so he relies a lot on a FB's decoy runs to penetrate the defense, and he lacks that "instinct" that certain players have (Hazard, Dembele, Neymar) that allow them to create something out of nothing on a consistent basis. Hakim is gone so now he's struggling. Man Utd fans were expecting him to be their Hazard on the right wing, and with hoe incohesive their squad is he definitely would have underperformed.
For Havertz German fans warned PL fans about him a lot. They all claim Havertz to be the best German talent in the past few years, but specifically warned Chelsea fans that he plays behind the striker and likes to make runs into the box; in other words, he's not a centre midfielder. Chelsea bought and they're playing him as a no.8, and lo and behold he's under-perfoming and now they don't know what to do with him.
Pogba back in Juventus played as a no. 8 in front of two midfielders, a CM and a DM. In Man Utd's crippled midfield, for many years he was expected to be a box to box, deep lying playmaker, a defensive midfielder, a controller, and a creator.
It goes on.
The reality hasn't changed. Clubs should go for players that specifically fit the needs that they have, not go for players for needs they don't have and expect them to play completely differently than they were in their previous clubs. Whether it's for 5 million euros or 140m euros. Many of the 80m> flops failed because of the clubs inability to scout properly, and just really going after a shiny new toy hoping it'd transform into their hazard, neymar, or de bruyne. It just doesn't work like that.
All the big money buys do not certify that big money transfers are certified to flop (they aren't), it's that those sanctioning these clubs' transfers are pretty much clueless about the players they want to sign.