It's surprisingly safe - but you have to do it in spurts. In fact I did a full-sized roving that way this afternoon as well. With samples just microwave on high for 30 seconds (make sure they're quite wet/submerged in water) and once they start bubbling like mad stop the microwave. For yarn you want it submerged as well, and I microwave in one minute increments, letting it cool for three minutes in between. By 5 minutes of divided heating time the yarn has usually reached 180 degrees by my temperature probe and then I let it cool naturally and rinse/dry like any other dye job.

It works remarkably well all things considered, and is very fast, but it is not a dye method you want to walk away from :)