You're almost guaranteed to get block decay, in addition to debuffs and other effects.
That said, I don't use Mystcraft in regular play myself, so I can't tell you about more than a small handful of experiments. You might get lucky and get a world without block decay, or not. Can't say for sure. But I can tell you that every single world I've ever gotten was utterly nasty. Useful if you have a ginormous tank back home and you bring several pumps, a liquid tesseract and an energy cell with you, so you can get a couple thousand buckets really quick and then get the hell outta there. But certainly nothing you want (or can) have a permanent installation in.
