#1 It puts Premier League clubs at a big disadvantage
Whereas the window closes in 3 days' time in the Premier League, it remains wide open elsewhere in Europe. This means other clubs outside England can continue buying players in England while English clubs cannot buy any players in replacement. This is a terrible situation. We all know the allure of playing for a club like Real Madrid.
We also know that the Spanish giants have sold Cristiano Ronaldo and could be in the market for a replacement. If they came out with all guns blazing to sign a Premier League player, say, Harry Kane or Eden Hazard, and the player was determined to force a move, the selling club could be forced to give in, sell the player without having the opportunity to replace them.
It is also clear that as the market edges closer and closer to a conclusion, some deals that weren't available suddenly become available. For Instance, at the start of the 2013 offseason, Mesut Ozil wasn't available for transfer, but he suddenly became available when it became clear that Gareth Bale was transferring from Tottenham Hotspur to Real Madrid.
This presented Arsenal with an opportunity they hadn't seen coming, landing the German playmaker in a £42.5m move. This, therefore, puts European clubs, with whom English clubs compete in the Champions League and the Europa League at an advantage of pulling off major deals English clubs would have taken advantage of had they been on the same wavelength.