One of my biggest issues with Returnal was how far it leaned into the rouge part of rougelites without offering engaging replayability. Your permanent upgrades offered relatively little strength. This meant if, for example, you wanted to beat level 3, the game structurally encouraged starting at levels 1 or 2 to get extra powered up. Saros leans much harder into the lite side of the genre. You have a skill tree that significantly upgrades your strength between runs, when rerunning levels the layouts are less random, and the game forcibly caps your power level so you don't one-shot the entire game (like I accidentally did with Returnal).
While it smooths out the engaging rough edges of Returnal, I certainly enjoyed Saros's structure more. But they inverted the pacing issues. With Returnal, I felt either slightly below or way over the power level the game expected of me. This resulted in runs where: 1a. I choked early on and died, 1b. Barely made it to the boss and died , or 2. Got OP and cleared multiple levels/bosses on my first try. There was almost certainly some of 1c. happening as well (made it to the boss and outskilled them), but I mostly remember 1a and 2.
I think in a rouguelite you want 1a to lead to 1b which becomes 1c. I think it is bad for Returnal that my memories are of 1a and 2. (In a game that truly targets replaybility, like Slay the Spire or Binding of Isaac, having the occasional 2 is great. Returnal's rogue/campaign fusion means the devs did probably not desire 2, as evidenced by the heavy-handed power caps in Saros, and the fact they will forcibly boot you back to the hub after certain bosses.)
With Saros, this 1a to 1c path is mostly intact, helped along via the meta-progression and player skill growth. But turns an annoyance from Returnal into a glaring flaw: because Saros lacks the diverse build options you'd expect from the genre, and because of weirdly similar level structure between runs you wouldn't expect from the genre, the game gets boring. Returnal's higher difficulty and my power level being pingponged around sufficiently distracted me from how same-y runs felt. And it felt like Returnal's level chunks for proc generation were smaller but more numerous, which I prefer over Saros's larger but fewer level chunks.
I watched a video review of Elden Ring: Nightreign from Scott Jund awhile ago. Something he pinpointed with that game was how its roguelite structure impeded its best quality: boss fights. I feel this with Saros as well. The boss fights were far and away the most engaging part of the game. But having to slog through 20ish minutes of a level that's 80% the same as it was last time to get there really undercuts this strength. I wish they relocated some of the difficulty in the levels to the bosses, but made the run-backs less excruciating. Or something like that.
I don't even want to talk about the story. Returnal actually pissed me off; Saros is just annoying. What I'll say is this: I think these games are too in love with themselves. They are not as fun as they want to be, their stories are much less impactful than they think they are.