Just experienced one freeze in 10.11.5 final.
Using FCPX, leave it on overnight to render / export 4x 4K video last night. On this morning, all I have is a black screen and a spining wheel. Luckily all video export successfully. However, by checking the load average history, there was something demanding 450 threads for 7 hours.
I realise that the system occationally has this kind of demand for a short period of time, which may freeze the Mac for few seconds, but it's the 1st time I've seen that occupy the CPU 100% for more than 7 hours. However, i can still login my Mac and move the mouse pointer via remote control (same black screen on the client side), the system still respond to hot key, I can use Command + Control + power button to restart the Mac, no need to perform a force shut down. So, not 100% frozen.