Today BetaFPV released a statement about the failure rate of the Beta85x. They have identified the ESC as the source of the problem, more specifically a component that was not soldered properly.
From 2000 models from the first batch at least 100 seem to be affected, the number could be much higher since some of them might still sit on a retailers shelf and just wait to burst in flames.
The problem seems to mostly show on 4S with a more aggressive flying style (going fast, lots of dives). In their tests BetaFPV found that there is a 50% failure rate on 4S. The errors could not be reproduced in their test when using 3S, although on Facebook some people report that their quadcopter did go up in flames while using 3S, BetaFPV states that this could only occur due to prior damage.
Betas recommendation is to not run the Beta85x with 4S.
Unfortunately a lot of customers are not happy about this statement - they bought the copter for its ability to run on 4S. Some customers are also not amused about having bought the 4S batteries that were recommended by BetaFPV and are now not able to use them, or risk losing their warranty when using them.
In their tests Beta found, that there were two scenarios of failure, the dramatic one, where the quad goes up in flames and the less dramatic one, where the copter simply falls out of the sky and stops working. In the latter case they found a component to come lose after the solder melt.
They further discovered that the ESC also kills the motors when it dies.
BetaFPV says, that the problem exists because of incomplete testing. They did not have a proper rig to test the ESC/Motor combination and in their manual tests they only flew the copter in slow speeds. This is very unfortunate, but it is nice, that they admit an error on their side - I am sure they will not repeat this mistake a second time.
BetaFPV claims to take 100% responsibility, they offer solutions for dealers and customers that ordered from them directly.
According to their statement, they put measurements in place that will not allow such a thing to happen again.
Personally I wish them the best, I own a Beta75x myself and am very happy with it. I also wanted to pull the trigger on the Beta85x HD version.
The second batch will only go up for sale once the cause is confirmed and proper testing was done.