I agree that they were decent, but remember that we have seen Liverpool lose to Atalanta 3-0 when they were the leaders in the league, that should tell you how hard these competitions are. Of course they dropped to third place later on but that doesn't change the fact that we had them at the top during the time of the loss, a team leading premier league would be expected to beat most teams in the world, including UCL teams too, maybe not Real Madrid, but there aren't that many teams leader of premier league could lose to, and yet they lost to Atalanta.
So saying that United is good right now is true but also they could lose is once again true as well. Just because they look good, doesn't mean that they are going to win it, so much so that even if they end up being somehow incredible and win the premier league and shock the whole world, that still doesn't mean that they will win this one too. You have to play game by game to see how it will end up.
Liverpool's defeat in the Europa League against Atalanta last season was quite surprising, not only that because they also lost with a fairly big score at that time. So just like the others, the Europa League is also quite difficult to predict.
Talking about Manchester United, actually every season the squad they have never lacks good players, but for some reason the players can't show their best abilities.
I think this has something to do with the strategy that is applied, maybe it's not suitable for the Manchester United players so they can't show their best performance.