Maybe I just get around too much, but there is nothing in ptba that says your flavor cannot do many of the things Stuff said it doesn't do.
Mathmatically 3d6 roll under is not that different than 2d6 roll over. I have heard GM's do all sorts of add 1, subtract 2, roll 3 dice with advantage/disadvantage, etc. Heck Iron kingdoms RPG is based off very similar math in a "crunchier" ruleset (although you can get up to rediculoisly high numbers of Xd6 in that).
Ultimately systems based on percentiles and d20 are really easy to calculate on the fly and lend themselves well to oh just roll x or better.
Still need to get gurps core books and read through that mess someday cover to cover (seems to be what I do), the systems I have a hard time with at the moment (aka have not gone on troll dice roller or any dice and played with the math a lot) are the XkY systems and the one roll engine mechanics.