Barcelona vs Manchester United match has now become the main object of Europa League discussion. But I still have little fear for clubs like PSV, Roma, bayern leverkusen, midtjylland and Monaco. These clubs are doing well in their league but their chances of winning their league is much quite narrow compared to their chances of winning the Europa League, for that they might show much more curiosity in winning Europa league matches than clubs like Barcelona and Arsenal. Therefore one should not underrate them in Europa league matches.
Yes such teams allways pose a threat just like frankfurt did last season and shocked the world by winning europa league with just average players so barcelona and man united need to be carefull i think any of the teams you metioned could pose a threat especially psv, monaco and leverkusen, roma performance has been quite poor this season but they have mourinho as a coach which with his experience can do wonders.
The biggest weakness of the Barcelona team right now is their midfield and some deficiencies in defense. If the team leaders add some good midfielders and defense teams then hopefully Barcelona team will be very strong.