I've been using an Edelrid Microjoule (and the larger Megajoule) belay device for several months now. Really happy with it, up until yesterday, when my Microjoule broke while rappelling. The thumb release just broke off. Kind of worrisome quality control on a device that is safety critical.
You pulled the tumb release while rappeling? You're not supposed to do that, the instructions (which you read, right?) clearly tell you to use a crab when rappeling with the autoblock function. Kind of worrisome attention to safety critical maneuvers you're showing there.
Also, this failure mode has been documented a few times, I'm surprised you didn't notice/google before posting here.
FWIW, Edelrid are currently revising the Joules. According to informal communications reported by major webshop they are addressing the issue in the revision (among other things) since a handful(!) of people have managed to break their devices this way.
Finally, the thumb release breaking while rappeling would have me shitting my pants, but upon calm reflection there is no danger, as the device blocks and can be released with a carabiner (as per instructions) or IIRC even by manual tilting.
The Jul design is faulty; a handful of known internet reports of failure is way too many. The thumb release is still a "release," and the fact that it may pop off when using it "incorrectly" for rappelling is unacceptable.
In addition, the stainless-steel Jul is apparently gouging aluminum carabiners, and there is a rumor that Edelrid will come out with a stainless-steel belay carabiner to pair with the Jul.
I'd stay away from this gadget until Edelrid gets all the problems sorted out.
Edelrid customer service response excerpt from the post: (Google translated from the German.)
"From the first batches of a total of 8 units have us actually sent back where the wire was torn out.
In half of the cases we have to assume was that tries after Geoquest articles forcibly pull out the wire.
During rest was actually the compression of the wire obviously not sufficiently strong.
since the units are assembled here in Isny, we were able to respond immediately and have the compression revised. Currently, the wires have an average separation value of 4.5 kN. When used correctly in accordance with instructions for use appear on the wire forces of up to 0.2 kN.
since it is the wire not a safety-related component and reliable brakes the machine easily without a wire and can be used for rappelling, we see where there is no danger.