Avoiding betting on Manchester United games because of their surprise results I don't think is the best thing to do if you're actually a strategic gambler. Personally, I avoid the option of direct winning whenever I'm staking on a game involving Manchester United rather I go for number of goals options. For instance, they travel to Brighton's home ground this weekend and instead of going for a straight win for either of the teams, I've gone for the option both teams will score. Manchester United under Erik ten Hag are always surprising with their results and for a team with such rate of inconsistency in results, it's always better to go for the option of goals than that of a straight win.
I think United have lost so much of their top team image in recent years that bookmakers are now at least fairly priced (or even underpriced haha) so there's really nothing wrong with betting on United (especially against the favourites).
As for the BTTS bet, here, on the contrary, it seems dangerous to me - Onana, on the one hand, is unstable and can let you down at any moment, but on the other hand, he seems to be the leader (definitely in the top) in the number of dry games.
We all know how inconsistent Manchester United was last season, however we need to understand that this is a new season and it will not be fair to measure them with the story of last season. This is the reason I do not always advise people to start gambling in the early time of the season. Maybe gamblers need small time to understand how the season is flowing before they begin to gamble. Talking from the statistics of this season, you understand that Brighton has scored more goal than Manchester united and this means that they will be able to score at least one goal against Manchester United. The real matter should be if Manchester United will be able to score Brighton. If both cases are met that means both teams to score is a good option for Manchester United versus Brighton