going on last season
United out losing to basle
Spurs out of a group with part timers in the EL were hammered by spanish opposition the previous season after struggling to qualify against young boys
City flapped
Arsenal ditto
Only chelsea though sheer fortune represented the prem
Foreign teams put more effort into the europa league and rightly so, the issue with this country is the obsession with the champions league which actually means teams don't put out their best players for the europa league or have their eye elsewhere.
When was the last time a team apart from barca or madrid won the la liga? 2003-04, so it goes to reason that the clubs in spain will do better in the europa league.
Also the last time a team aside from barca or real won back to back titles was 1983-85 (nearly 30 years) which was Athletic de Bilbao.
Overall the premier league is a stronger league but the la liga is more technical.