I had nothing but persistent problems with my setup: sources - Radiance - HD621 - G68. I tried everything - switched Radiance and HD621, multiple FW updates, different brands of HDMI cables, different lengths of cables, tweaking audio EDID settings - all to no avail.
I've been through every revolution of the Lumagen scalers and am currently using their 2042. No problems with any of them and my HD621...
The HD621 seems like a very stable and solid implementation.
If there were any issues I'd be pointing my finger at Lumagen, particularly as they are constantly evolving and usually resolve (temporary) issues with their own hardware, and with third parties, rapidly.