He's a mediocre player (we can have deep conversation re what his weaknesses are, and they haven't changed), signing mediocre players always backfires. Additionally we already have 2 players in that position (Porro/Spence).
Fair, even though I still think it's a sign of his mental space, I very much doubt he's is thinking of rebuilding his career at Spurs or Saudi Sportswashing Machine, he's thinking he's going up from United (but hey, could be worng)
The risk with Rashford is not the money, even though we would be nuts to make him our best paid player, which would cause issues with player like Deki/Romero, etc. The risk is he is not a fighter, the last thing, the very last thing this club needs is another player that goes missing, that isn't up for the fight every game (something Rashford has never been).