They are more difficult than returnal imo.
Returnal suffer from inverse armoured core 6 syndrome, the levels are hard, the bosses, not so much (mostly because i noticed that they deal less damage per single hit than your average trash mob, logic when you think that there are hundreds of projectiles during boss fights and it is nearly impossible to avoid all of them).
Like i was overwhelmed the first couple of times because i'm shit at bullet hells shooter but then i noticed that i could tank some hits...
And the more you upgrade the weapons, the easier the game is, some weapons literally melt health bars.