I'll say the circuit boards are replaced *way* more than needed- back in the days of the Suburban NT series and Hydroflame DC82 series, the circuit board was next to the fire box. This literally fried them. Dinosaur came out with a much higher quality board- fiberglass with plated through holes. It would last longer, but even Dinosaur finally made a board saver which relocated the board outside the furnace cabinet.
On today's furnaces (except the NT series- still), the board is in the return air flow, which keeps it cool. I used to replace a board a week- now I replace a board a season. They just do not go bad that often.
