Across eight solder clinics we have seen roughly two hundred non-working boards. The distribution is remarkably stable.
About half are power: a supply that cannot deliver the current, a regulator running hot, or a board browning out the moment a motor starts. About a fifth are physical connections, usually a cold joint that everyone standing nearby can see and the owner cannot. The rest are genuinely code, and a handful are dead components.
So the clinic order is power, connections, code. It feels insulting when you have spent a weekend on the firmware. It is right about seventy percent of the time.