One ongoing and most popular theory is that the devil fruit eaters can not reach the Laughtale because we saw Toki and Buggy get sick. That is totally possible but it does not apply to the Straw Hats.
As the treasure or the Laughtale existed because Joyboy left something on it. Luffy is naturally not the one who would be sick on his journey.
Robin belongs to the Ohara clan, the archaeologists so it is impossible for the people from that land to be restricted in a place where all answers of history are.
Chopper, despite eating the devil fruit is a reindeer, hence the theory that he won't get sick as he is an animal who ate the devil fruit.
Brook naturally is not going to be sick because he is not someone who can get sick I mean I doubt a skeleton would have high fever (That was how Toki and Buggy got sick)
I'll continue my thoughts on other theories later..