But I never see bad solder on this parts.
Those. There's 3 of them (U13, U14, and U15) per hash board. My friend and I've seen hundreds (if not thousands) of them under a solder rework microscope. They all looked good from almost every angle. Touching them up all of them usually fixed a HW error problem (Once we narrowed it down to the right hash board). I'm not going to guarantee that it will fix a HW error problem. I'm just going to state that there's a good probability that it will fix a HW error problem.
But observe interesting thing, HW percentage are depends from length and quality ribbon cable. Maybe in conjunction with solder quality on buffers ?