I guess where I disagree on the story tone there is that USUM aren't light in and of themselves, but they feel a bit less dark from the previous games.
My basis for this primarily pertains to the story climax, as S/M is smaller in scope but also feels more grounded in its subject matter. It's not about stopping an Apocalyptic event, just dealing with the fallout of a broken and abusive family life, without a lot of innuendo or obfuscation for younger players. Rainbow Rocket gets some "theme park" vibes to me given the whole premise is "here's a throwback to all the big villain bosses", even if contextually they are taken seriously, on a meta level there's a concession to having them there at all.
The resolution also sort of gets me. S/M Lillie and Lusamine start a path to healing, but things aren't "fixed" as soon as you beat the antagonist, with Lusamine still very sick (physically and possible mentally), Gladion's side of the issue not necessarily resolved, and Lillie not being able to stay with her friends or found family in an effort to help her Mom in spite of everything. US/UM by comparison has a conflict that can be and sort of IS resolved as soon as you defeat Ultra Necrozma, since the scars don't run emotionally deep, and even Rainbow Rocket, despite escalating the conflict, is very impersonal and disconnected from the main story one.
That's the main thing I think makes S/M stick out to people: The consequences don't end with the Crisis the way they do in most games (Gen 1-4 the teams kind of disband after you stop their big gambit, Gen 5 leaves N as a hanging thread which similarly made the character stick with players), which makes the S/M plot feel less idealized or like a "means to an end" to contextualize the gameplay journey, which I will say even about several gens I like such as Gens 3-4 and a lot of SV's main campaign paths.