#2 Emery is maximizing the potential of the team
When Emery was at Paris Saint German, he had the luxury of true wingers, that is players whose natural position was from the wings. He had players like Angel Di Maria, Neymar, and even current Tottenham winger Lucas Moura.
These options allowed the team's wing play to be very effective, with the likes of Serge Aurier also bombing forward as well.
The present Arsenal team lacks proper wingers. Aubameyang is not a proper attacking winger, neither is Henrikh Mkhitaryan. The only player in the attacking position who comes close is Alex Iwobi.
Unai Emery has preferred to play a 4-1-2-3 or 4-2-3-1 or even a 4-4-2 formation to accommodate wingers who can provide support defensively for the full backs when the team does not have the ball.
As a result, when playing teams who are technically good on the ball, as seen in Arsenal's recent games against Bournemouth and Tottenham, he has decided to go for a back three to give room for the full backs to join the attack wholeheartedly.
Sead Kolasinac and Hector Bellerin are providing the necessary width in attacking positions for Arsenal. You just have the feeling that Unai Emery needs two transfer windows to really make this team his own, and they will be very difficult to beat.