When I think of the worst whiskies I have ever tasted, no bourbon is anywhere near hitting the list.
The worst bourbon I have tasted is something like the regular Jack Daniels, and it is not that bad when compared to what is on the shelf out there otherwise. (#JDisabourbon)
There is a reason for this. Both scotch whisky and bourbon are regulated, and both have more heavily regulated subgroups (straight bourbons and single malts).
Most of what is bottled bourbonwise are straight bourbons, so much that the regulations for straight bourbons are often spoken of as the general rules for bourbons. Just take a tour on a bourbon distillery if you don't believe me. The two main rules for straight bourbons are minimum 2 years in the barrel and no additives. One of the main and most important rules for bourbons in general is that it must be matured on new virgin oak barrels. Once the barrel has been used, it can not be used for filling new spirit in again, if you want to make bourbon. Not much to play around with here, and it give bourbon a great deal of consistency compared to barrels. The difference in maturation for bourbons comes more from warehouse location and age more than type of barrel.
In Scotland casks are used and reused as many times as the producer wishes. A cheap blend consist of a lot of grain whisky that has been matured on almost dead casks for three years. A cheap bourbon is probably both older and has been on better barrels than a similar priced scotch.
In Scotland virgin oak is used, ex-bourbon barrels, ex-sherry casks, ex-whatever-you-can-think-of really. Then the barrels and casks are reused for refills, and often when the casks are used for the 4th and 5th time it's just containers for grain whisky maturation. And then there is also a lot of finishes going on, where whisky is matured on one kind of casks and then transfered to another type.
This gives scotch a huge varitation in whisky produced and the quality of the whisky. The differences in maturation for scotch comes from cask type and age, not the actual warehouse location.
I do know I simplified things a bit. Bourbon finishes do exist and a few bottles of single malts also exist where warehouse location is emphasized, but in the big picture these bottlings can be neglected
But the conclusion is that it's actual quite hard to find a really bad bourbon out there. Finding bad scotch is easy
PS When it comes to my top list of whiskies it's heavily dominated by scotch, but I have bourbons and ryes on that list as well. When it comes to high-end bottlings Scotland is much more prolific. As bourbon is getting much more popular these years I expect to see a lot more from US. If they can keep up with stocks needed..