It's really hard to be bad honestly. By default you're supposed to reroll when your modifiers add up to 0 or lower or if your highest score is 13. The lowest you can be using the rules in the PHB in your best stat is a +2 unmodified which is only 2 less than the best possible. On top of that, the chances of actually rolling bad AND not having to reroll is pretty low because rolling the average is a reroll. The worst roll you can have is a 14/10/10/10/10/8 with a +1 total modifier. Which tbh isn't that far from a point buy of 18/14/10/9/8/8 which has a total of +4. The most efficient point buy is a 14/14/14/14/9/8 with a +7. Your worst possible roll is +2 behind in two stats from the min/maxed point buy while being +1 ahead in one stat. In the efficient buy it's only +2 behind in 3 stats that aren't the important one while still having a +1 in at least one stat. While I can't actually comment on the commonality of rolling extremely high, I've only ever experienced or seen someone else experience a triple 15+ roll a handful of times where the other 3 scores don't cause a reroll. Most of the time I see rolls that are pretty close to a 25 point buy