A while back I wrote a post about why I disliked the ability check mechanic recommended in Old School Essentials and analyzed various alternative methods. I wasted a lot of words on it. Here is a graph that shows you the differences between several methods and you can draw your own conclusions.
Here's the gist of the numbers and the graph at the top. It represents the chances of success for different methods of rolling under ability. I.e. roll d100 under ability, roll d20 under ability, or roll 3d6 under ability. These chances are compared to the most common class-based skill method of x-in-6 chance, with a 2-in-6 chance being by far the most common.
- Top green row is the character's ability score.
- The next three rows show the actual chances of rolling under the corresponding ability score by dice method.
- The graph visualizes the data in comparison to the 2-in-6 chance that is common among class abilities.
After playing around with skill rolls for a while at the table, my conclusion is that all skill rolls should use the same mechanic. You can't have two different mechanics without having radically different whiff-factors. Specifically, it is far easier to succeed at a roll d20-under-ability (assuming your ability is a 7 or better, than it is to succeed at a 2-in-6 roll. Thus allowing characters to use the ability check in a situation where some classes would have to go for the class roll, seems out of scale and, frankly, "unfair."
My preference would be to use an x-in-6 chance for everything, with no ability bonus to the roll.
IF the d20-under method is used, it should only be used for skills that "anyone could do" and which are not a defined class skill. Class skills that seem like anyone could try them, i.e. "hide in shadows," need to be read as potent rather than mundane skills. In other words, the hide in shadows ability might allow a thief to disappear in a brightly lit warehouse (in the shadow of a crate perhaps) or even behind another, larger character in a hallway. Characters without this ability couldn't even attempt such subterfuge, and should only be able to hide in shadows when it's really obvious that anyone could do it, e.g. standing in a side alley at night.