I'm really interested in this. I like the idea of all the numbers in the game being on the same scale, conceptually. And I also like the simplified arithmetic that can potentially speed up decision making in play.
Now CoC 7e solved the scale problem to an extent by bringing the stats into line with D100. But hitpoints for humans, for example, remain in the realms of the D20. I think the simpler solution is to bring all the mechanics back into the D20 range. Keep the 3D6 basic stats and the relative hit points and such, but divide all the percentile chances by 5.
I played a bit with the idea of rolling high, rather than rolling low. But the calculation for the equivalent target to hit are simply a headache. You could use tables but no one needs that. The benefits of getting regular players of D20 based games to more easily know what they're rolling are outweighed by the extra costs of determining the different difficulty levels needed to succeed.
It's very simple to use a D20 for percentile rolls with all skills scaled accordingly, and to roll low. 60% chance to hit, 30% hard, 15% extreme (or variations depending on the system) becomes 12, 6, 3 - roll under on a D20. Every +5% bonus or penalty is a +1 or -1. Every successful skill increase results in a +1.
Bonus or penalty dice approach simply works as in DnD 5e - except take the lower rather than the higher roll or vice versa if penalty.
Ultimately the difference is small and mostly one of feel since, even if the arithmetic is slightly easier, it's no like adding any two double digits is all that taxing. But i can make things a little easier, especially for calculating halves and quarters (etc.) on the fly (and no annoying +23% bonuses to juggle in your head to slow down the pace of a dramatic scene). And I like the conceptual aesthetic of bringing everything onto a similar numeric range within the mechanics - your character has a CON of 15, HP of 15 probably, and her skill with a battleaxe is 11.
WRT critical success, it's a very fair point. I think you just have to abandon the idea of giving people creeping extra percentiles as their skill goes up OR include a second dice roll on a roll of 20. Not ideal. It is the biggest problem with converting to D20.
I'll reiterate, I think the difference is small, but it could create a little bit of a smoother play experience that I'd like to try out.