"Is there anything the experimenters could ask for but never get?"
Well, I'd guess they couldn't get anything incompatible with the actual laws of physics, like FTL (if that's impossible).
Even so, the problem with this scenario as I see it is that there wouldn't just be the three types of world "wish granted"/"machine failed, nothing happened"/"earth disintegrated", but quite likely partial "successes" as well, where the machine failed in disastrous but not apocalyptic ways, e.g. only a third of humanity died. Whether or not that's ethical I'm not sure, but those surviving folks in the "a third of humanity died" world sure would be upset.
"Is there anything the experimenters could ask for but never get?"
Well, I'd guess they couldn't get anything incompatible with the actual laws of physics, like FTL (if that's impossible). Even so, the problem with this scenario as I see it is that there wouldn't just be the three types of world "wish granted"/"machine failed, nothing happened"/"earth disintegrated", but quite likely partial "successes" as well, where the machine failed in disastrous but not apocalyptic ways, e.g. only a third of humanity died. Whether or not that's ethical I'm not sure, but those surviving folks in the "a third of humanity died" world sure would be upset.