Goodman furnace - intermittantly works (80% of time works, then one blink (ignition failure)
The startup of a gas furnace usually sees the controller moving through a series of states. The controller would verify that high temperature limit switch is not activated, the exhaust pressure sensor shows no pressure, then switch on the inducer fan, then verify that the exhaust pressure shows pressure, then turn on the igniter. I don't know if any common controllers verify the igniter (current consumption, heat produced, etc) or not. After a delay the gas valve is opened. As long as the gas valve remains open the controller continuously looks for the flame proving sensor to verify that the burner is still burning.

If your furnace always ignites when there's a call for heat but subsequently extinguishes before the call for heat was satisfied then the problem could be anywhere: flame sensing, exhaust pressure sensing, high limit cutout.. flame sensing is probably the most likely, though, because arguably it experiences the most extreme operating conditions.

On the other hand, if it sometimes doesn't ignite at all, that's more likely to be an igniter problem than a flame proving problem. I agree it seems most sensible that an igniter might completely fail, but I guess there must be some intermittent failure modes possible too. You could remove it and visually inspect for cracks or other signs of failure, but be prepared to deal with the possibility that this might push it from partial failure to complete failure!