Ultimate team was really fun and addicting in the beginning because it was interesting to see the varied teams people had at the time. Before the majority of the people were able to get Casillas, Torres, Villa, Benzema, Henry, Cristiano Ronaldo etc with 99 stats. 80% of the teams you'll face are just Spanish, French, and Brasilian National teams because having players from the same nation grants a chemistry bonus. In the beginning, it was fun to hunt bargains in the trade/market place. Since the market is oversatured, its not difficult now.
The flaw of the damn thing is that with 99 shooting, finishing, and acceleration. There is no defense to be played. Most of the time, its a damn goal scoring fest. I literally take 30-40 yard shots with 99 shooting/finishing Karim Benzema and I score 70% of the time.
Then, theres gameplay cards which grant advantages to the player for one game. For example, you can play a card that slows down their striker or defender making it easy for you to bash the other team. I personally do not like gameplay cards. My biggest gripe with Ultimate team is that it takes away from skill and just places more importance on players,player stats, and gameplay cards.