Opticallimits

The thing is - the (manual) focus confirmation dot has a very wide "in focus" range. I wouldn't call it exactly accurate near the "edges" of that range. This may be Okay for slow speed, wide angle lenses but that's about it - in my opinion.

Not sure, if faster lenses could get better focus accuracy because of shallower DoF. I think it depends on the lens' overall performance. The more CA wide open, the worse the accuracy?

I have some different experiences with it. 

Just two examples, using focus trap (which works on focus confirmation).

Ultra Micro Nikkor 55mm f2. 

 

I have quite a few other lenses which are pretty good with focus confirmation too, including a 55mm f1.2.