An acquaintance near Rochester, Minnesota, runs a successful business making goat's milk soap. For a sense of the volume of business she does, she churned out 800 bars for special orders this past Christmas season in addition to her regular wholesale and retail accounts. Every batch she makes is done in a blender. I've watched her in action, and she's really good at what she does. Maybe it's not for every person or every recipe, but it is a reasonable soapmaking technique for the right situation.
edit -- Clean it out like you would clean any soapmaking equipment. Fill with hot water and soap, let it soak for a bit, run the blender a few seconds, rinse with more hot water. No big deal. She buys cheap $20 blenders that have blade assemblies with simple bushings, not the fancier ones with ball bearings. The lye eventually gets into the bearings and causes them to seize up. Which argues that a soap-making blender probably shouldn't be also used for food.