I have seen the critique before that Gandalf is describing Valinor, men have an unknown fate when they die, and Hobbits are technically a branch of men. Meaning that Gandalf's speech is wrong since Pippin would not have seen the far green country.
I think this misses the thematic point, but there is one detail to note just for fun. We know from Beren and Lúthien that at least in some cases, men do temporarily go to the Halls of Mandos after death for a short time before whatever Eru has prepared for them. So actually by the strictest purist standard it can still fit canon. (Maybe Mandos has windows with a view of the white shores, who knows?

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