Well, I should give the specific examples of what I mean by 'hardier' - Tuor claims that he is near death from starvation/exposure, but that perhaps Voronwë is not. Is that an elf/mortal difference, or is Voronwë one of the Calaquendi, and does Tuor even accurately predict how hardy Voronwë is? He gets some lembas out of it, so all goes well in the end, but it's a rather slender thread to hold up the entire idea of great elvish endurance.
Another example that is more implied than explicitly stated is that Maedhros seems to have hung by his wrist from a rock for *years* without dying. Sure, he was freshly come from Valinor, but that sounds like a serious plight to be in. Are we to assume that Morgoth was sending out slaves with ladders to feed his prisoner? Was there magic involved, so that Maedhros was trapped in some sort of suspended animation rather than age/deteriorate like normal? Was Maedhros actually stuffed in a dungeon for most of that time, and just hung out on the cliff as a final punishment days before Fingon managed to rescue him? Who knows?
Other statements on the matter are mostly generic, about the Light of the Trees having been so recently shining on their faces, but that could mean a lot of things when it comes to practical matters like how strong their muscles were. It's...tricky.
* The sun rose for the first time while he was up there, so just how many years is a curious question. But even so...years.