Generally, whether a mechanic is good or bad comes down to how complex the math is to figure out. The easier, the better. The harder, the worse. Now, I know somebody out there will disagree.
In short: shut up, you're an idiot.
In long, everything EVERYTHING comes down to percentages. That's right, it's not just BRP or Call of Cthulhu. In fact they do it shitty even though they use flat percentages. let me explain:
Roll and add vs TN is better than roll under. You could roll vs ability scores a bunch of ways but the general normal way is the following:
Roll 1d20, try to get under your score. You make it you win. If you have a penalty to your roll, it's either a positive modifier to the roll, or a negative modifier to your score prior to the roll. Really it's a pain in the ass to keep it straight. Because "bonus to roll" doesn't mean a positive modifier to a roll.
Here's a mechanically identical alternative that is better:
Roll 1d20 + ability score (or modifier), try to meet or beat TN 21. Want it harder? Add to the TN. Want a bonus? Add to either the ability score or the roll, it doesn't make a damn difference. This makes logical sense because increasing difficulty increases difficulty, and "bonuses" to roll are all plusses, negatives to roll are all negatives to result.
So, everybody shut the fuck up about roll-under already.
