To be fair, Wanyama was pretty up and down at Soton last year - their fans were laying into him, suggesting that he'd gotten lazy and that he only tried in the big games (so he could put himself in the shop window). He comes to us, many (although by no means all) Southampton fans are glad to see the back of him, and then he immediately goes from being an on-and-off red card magnet with limited passing ability to being the second-best enforcer in the league behind Kante, a brutal physical specimen who allies excellent close control with magnificently precise, controlled aggression.
I think in Schneiderlin's case it's more to do with us just being very good on the day, because GOT seems to rate him really highly overall. I think, like Wanyama, he needed the move away from United to flourish - he's doing that at Everton, and good luck to him.