By failures things like vampires/bandits try and take over your mine. Or they dig into a vampire/falmer/dwemer/daedric ruin. Or a gang tries to extort protection money from you.
By critical success, things like finding rare gems, or tunnel into an abandoned ruin with leveled loot in it. (Think a treasure room you could even just add in un-enchanted rare armor types. Like nightingale looking armor, that has no enchantments)
Imo things like that would be really neat. If you make a count of the minerals the mine is generating, a variable other mods can check, other mods could use those values in building cities or etc.