Manchester United is an inconsistent team, and that is why betting on them is very difficult. I could see them beat Lyon 3-0, and I could see them lose 3-0, both of them are valid and possible. Not many games happen that way, for example none of the other matchups are like that, you can assume what will happen.

There is almost always a better team, and the better team will either easily win, or barely get a tie or a loss, but the better team won't get dominated, that is very rare. Or worst case, two teams will be close to each other, and either one of them will barely win or there will be a tie. With United, betting is impossible because you have literally zero clue what is going to happen with the game and you can't predict the result.