taking a stab at the whole valinor trauma rebirth tragedy thing
without getting into the various iterations of mandos throughout its development, the fact that it shares the etymological root with angband (angamando in quenya) or its use as both a prison/punishment place and as the temporary "afterlife" of elves, I think it should be obvious why "mandos cures everything" doesn't work for most people. it's simply narratively unsatisfying. we know that, technically, spirits are solitary in mandos and don't tend to interact with each other, and we know (iirc) that nienna does most of the healing work, when those spirits don't reflect on things by themselves.
healing in complete isolation might work one rare time, but it's otherwise simply not how healing works, how adjusting to a new life works. being way too in your own head is discouraged. healing all your traumas because a goddess did it via magic counseling gives, at best, uncanny vibes, at worst erases the struggle and journey of adjusting, with help, into the life you're actually living. so people either say that spirits can actually meet in mandos and figure things out among themselves, or subvert the narrative and have people come out of mandos either not truly healed or only partially so, and needing the real living feedback of society to exist within it again. a reading which allows mandos to still function as a recovery, but whose achivement is to "prepare" for the journey of spiritual healing, to bring elves back to a stage where they're able to face the circumstances that generated their trauma (aka the living, embodied world, and maybe more precisely even the people involved in it).
this barely touches on the grievances that dead elves might have with the guys who are running this show. this isn't just feanorian followers (or the exiles more at large) who renounced the valar's authority, it can also be the avari, who now either get valinor or they get valinor. it can be the falathrim, who wanted to go to valinor and lost the chance. it can be those sindar who were waiting for a full intervention from valinor, and it didn't come until earendil came around. it's hard to envision healing within a system when the system itself is what you take issue with. it requires a personal compromise, or an acceptance of the system's authority, and that's simply not always possible, nor can fanworks always easily tackle it — which is also why I think fics where living relatives "bully" or like, strongly entreat, the valar into releasing specific elves from death are popular. it's one way of giving that specific problem a solution, though it may in effect be unrealistic. it's less about realism (I for one don't believe the Valar would ever do that) and more about trying to find a way through wanting to see those characters heal without having to bend and accept the system and its authority.
which also brings me to what comes after and the necessary divide, real or perceived, between people who were always in valinor and people who returned to life after conflict.
to put it simply, making sweeping statements about whether amanyar elves can understand the trauma of exiles and other reborn elves is not possible and in itself pretty silly. even the amanyar themselves don't perceive their experiences of trauma and the darkening in the same way! the teleri refuse to set foot in beleriand despite their own kin being there, and despite the fact that noldor and vanyar embark on a valar-sanctioned war. it's pretty obvious that their own internal experiences and cultural understanding of the darkening or of valar authority is still vastly different, that even going by the imprecise and generalising divide of clan, that trauma was processed differently. or not processed at all.
and then, would those who fight the war of wrath understand the trauma of a continent-wide collapse? yeah, surely in a sense they can, they live through it. but can they understand it from the point of view of a sinda who had lived in beleriand all their life and didn't simply come here with the understanding that this was war? who saw their home be destroyed slowly and painfully, and in the end, when the saving arrives, it's a saving with such an immense and heartbreaking price? maybe they can empathise, maybe they can't. the darkening, by the time of the war of wrath, is no viable term of comparison. even among the living, this isn't cookie-cutter.
so what of those who die and return? I think it's obvious, in the text itself, that someone can go through a death, real of metaphorical, return to their old home which has itself gone through some considerable trauma, and realise that no matter if both you and your home have changed, both have bled, you're still unable to readjust to it the way others can. other people who were with you in your journey can integrate, they find old friends and loves who help them in this. you can't. I'm obviously talking about frodo.
it's not the same for everyone and it will never be. and I do feel as though the reading of valinor being in itself unable to take back people who went through trauma is a push-back against the idea that valinor must inherently be blissful, healing, and perfect; but the text presents us many situations where the environment of valinor plants the seeds of dissatisfaction; the fact that it doesn't work as neatly as it seems is at the core of the early conflict in The Silmarillion (even without pointing out stuff like: troubled people, Frodo included, go to the gardens of Lorien in search of that healing and peace of mind that the rest of the land can't actually provide. it's just a land. it's mostly free of toil because there's literal gods providing things, but it's just a land). valinor is not perfect, but its status as blessed realm invites a certain unease in many readers. I believe this unease leads easily to cotradictory and equally extreme positions, ranging from "no one would or should feel out of place after rebirth" to "actually no one would understand the trauma of someone who died and returned".
plus, of course, the obvious: someone's trauma, collective or individual, and how people process it, doesn't somehow erase someone else's and how they process it. the two things can come in conflict with one another, but they're not, like, mutually exclusive.