(very bad things can result from Men spending too much time in Valinor)
(src: this SFF answer)
Given the answer's poster prior history in lord-of-the-rings, my assumption is that the quoted claim is correct.
But what are those "bad things" which can result?
I'm not sure exactly what Shamshiel meant in that answer, but this is discussed briefly in The Silmarillion (emphasis mine):
'The Doom of the World,' [the Elves] said, 'One alone can change who made it. And were you so to voyage that escaping all deceits and snares you came indeed to Aman, the Blessed Realm, little would it profit you. For it is not the land of Manwë that makes its people deathless, but the Deathless that dwell therein have hallowed the land; and there you would but wither and grow weary the sooner, as moths in a light too strong and steadfast.'
The Silmaillion IV Akallabêth
Tolkien wrote more about this in an essay titled "Aman and Mortal Men":
[I]n Aman such a [mortal] creature would be a fleeting thing, the most swift-passing of all beasts. For his whole life would last little more than one half-year, and while all other living creatures would seem to him hardly to change, but to remain steadfast in life and joy with hope of endless years undimmed, he would rise and pass - even as upon Earth the grass may rise in spring and wither ere winter. Then he would become filled with envy, deeming himself a victim of injustice, being denied the graces given to all other things. He would not value what he had, but feeling that he was among the least and most despised of all creatures, he would grow soon to contemn his manhood, and hate those more richly endowed. He would not escape the fear and sorrow of his swift mortality that is his lot upon Earth, in Arda Marred, but would be burdened by it unbearably to the loss of all delight.
History of Middle-earth X Morgoth's Ring Part 5: "Myth's Transformed" Chapter XI "Aman and Mortal Men"
Essentially, their lives would appear shorter, because of the unchanging nature of everything around them; this would be rather psychologically distressing, as you can imagine, and it would eventually make them bitter, angry, and depressed. This is not a recipe for domestic bliss.
There are also more specific "Very Bad Things" that arise from having Ar-Pharazôn and his soldiers running around; that's dealt with a little more in Shamshiels's answer here, and my own answer here.
