Some of them in a first thought :

in TM (transfer market) you don't have some info about the player you want to buy
- no previous stats
- no info about the previous team. That's very critical because there is a category of players, the faster trainers (nordgen) where the team's name is an indicator.

- you loose some critical info about a game or an auction opponent, like the achievements and trophies.

- you can watch better a match of yours, you can have the main board with stats and formation and players and if your oppo also watching, if he 'll make any change in his formation, you can observe it easily (instead of scrolling to formations board)