I've filed the issue reports a few times, but it feels a bit hopeless. It is a really disappointing, frustrating issue. From OS X 10.6 and iOS 6 to OS X 10.10 and iOS 8, Apple's products went from 'it just works' to 'it just doesn't work'. The built-in screen just blinks and no external screen is detected. Macbook Air does not recognize devices connected through the HDMI adapter Turn off the HDMI device while making sure to keep your Macbook Air on. I've even had the SMC fail and had to reset it due to this issue, and it affects my work several times a month. Am on a work trip and cannot connect my Macbook Air to an HDMI screen any longer. When I switch inputs on my DisplayPort / HDMI display to view my PC desktop, it often crashes - Especially with the DP → HDMI setup/monitor, but also with the play DP cable (this is across two different test displays apart from the Ultrafine).If the laptop goes to sleep, I cross my fingers that it won't break. Unplugging and replugging the MacBook has worked for some users, but this isn’t always the case. If you are experiencing this problem, you can try a few solutions. In most cases, this problem results from an incompatible HDMI port on the MacBook. You are likely to encounter this problem if you use a MacBook. Other times, opening the lid crashes things. Why is MacBook HDMI Not Working Not being able to connect your MacBook Air to HDMI can be frustrating.I close the MacBook lid to save a bit of performance since I only use the two desktop displays, but sometimes that crashes things.It helped a bit to stop using the $300 Belkin dock I bought from apple for the Ultrafine, so it is only serving the third display - but issues abound.
I have had just terrible trouble with crashes as well on my brand new MacBook Pro this year, but connecting a LG Ultrafine and a Display Port (or HDMI via a DisplayPort → HDMI cable) display results in an unpredictable, crash-prone environment.