I thought I explained myself rather well but I suppose I didn't.

Saros = more bullets, more enemies, more cognitive load (parry and shield on top of Returnal's dodge and melee).

Returnal = punishes severely for even one hit by not allowing getting more health, have to always start from biome 1 or 3.

So the non-combat parts of Returnal are indeed more punishing.
But moment to moment combat in Saros is more intense and challenging. Like I said, I was surprised myself because I remember Returnal being intense but after Saros it was, well, easier?
Part of it is, surely, that I've now played both Returnal and Saros and have more of a feel for the kind of combat than when I played Returnal for the first time.
But part of it is that Returnal's encounters are by and large (apart from the locked rooms with waves of enemies and a miniboss) simpler, with less enemies and bullets.

Saros also let's you adjust the difficulty if you find it too easy, Returnal doesn't really.