I ran the Stans Flow wheels with some Butcher SX tires (tubeless) at Sea Otter. (I have been running tubeless downhill for years with Maxxis UST on Mavic UST). I thought it would be a nice, light setup for the mellow DH course. On one of the practice runs I had quite a moment when this setup failed me. Landing a jump into some ruts, the front tire blew straight off the rim. I heard a pop, looked down, the tire was instantly flopping around my handlebars. The rim dug into the ground, and I went OTB, giving my Alpinestars neck brace quite the test. I went over to the Specialized tent for some insight on this failure. They said the tires aren't certified UST - that's a risk I take running the tires tubeless. I also talked to Mavic who ,of course had a slightly biased response, saying the Stans aren't certified tubeless. I believe Mavic helped create the certification. Anyway, in the interest of safety/reliability I thought I would share this info.
Exactly my results with flow rims too. Since moving to a proper ust rim (crossmax sx, Easton haven carbons and a pair of Easton havocs) I've ridden a mixture of maxxis and specialized tyres tubeless and never had any issues.
My call is for hard riding the bead seats on the ztr rims just isn't up to the job.