It doesn't support standard UML diagrams but between sketch mode [1] [2] and icons [3] you may be pleasantly surprised. I personally use it to map out Kumu's own internal application structure and flows.
Full disclosure: I am the lead developer and cofounder of Kumu.
If enough people are interested I'm happy to do a webinar specifically for the HN crowd and do a deep dive on the technicals. Built on top of CouchDB and pretty neat stack overall.