RPG

So the Mophidius 2d20 system works like this. You have two numbers (and how you get those two numbers vary from game to game, for example it can be that each skill inherently has two stats like expertise and focus, or it can be that you get one number from the skill value and the other from a trait matching system). You roll some d20s, two by default but you can get more (for example, in Star Trek, you get more by using teamwork). Each die under one of the numbers is a success, each die under the other number is two successes.

So it's for all the disadvantages of a typical success-counter like Shadowrun, Vampire, Burning Wheel or MY0 (wonky and opaque probabilities, lots of separate dice to look at) and all the disadvantages of something like D&D, where the numbers you're dealing with are pretty big.

It is fiddly and cumbersome. Not my jam. The advantage is that you get a lot of "inputs" (single success threshold, double success threshold, and amount of dice) and "outputs" (successes and momentum), which is good is you're making the sort of proprietary "gamist" crunch that sells books. (Not that those games can't be fun.)