Manchester United and Tottenham are teams that have good quality overall and what happens in the domestic league will be different from what happens in the Europa League so it cannot be used as a reference and even in the last few seasons we often see teams that win the Europa League do not have a good ranking in the final standings of the domestic league maybe that will also happen to Manchester United or Tottenham.
For the opponents that will be faced in the quarter-finals I think all teams have equal quality and also in terms of percentage all teams have the same possibility to be able to win the match and advance to the semifinals.
Looking at the teams playing in the quarterfinals, I think only FK Bodø/Glimt and Rangers are underdogs. I was even surprised that FK Bodø/Glimt made it to the quarterfinals. For the rest of the teams I think they are on the same level. I don't think Manchester United and Tottenham will be the dominant teams in the quarterfinals even though they might be the top favorites. Lyon and Eintracht Frankfurt could surprise. On the other hand Lazio and Bilbao are not teams to be underestimated. I can't even predict which team has the biggest chance to become the UEL champion. Because as you said sometimes we can't see their league position to determine their level in the UEL.