Actually, I think the Workers examine the terrain options without considering government (so a mod that made forest more productive, by allowing it to be mined actually ended up with AI/autoworkers Foresting over all the Plains)
The problems seems to be that they don't consider government penalties
And they also don't Seem to consider the needs of the City.
I think the rule for Irrigate v. Mine in things that can get equal value from both is Irrigate if tile produces less than required food for a citizen, Mine if it produces More, and 50/50 either way if it requires either.
When I leave my workers on automate not replace this seems to be it, all the food bonus tiles are mined, and all the plains and desert are irrigated, and the regular grass lands are split about 50% each (assuming there was water available).