At first, I find Pinsir hard to caught (I took me over 50 loops on my first attempt). But after upgrading to Fine Styler, capturing it is a breeze.
The Pokemon that I have a lot of trouble catching is Drapion, but it's probably because my mouse is a bit weird then (I use emulator).
Bastiondon causes me some trouble, but it doesn't take much time until I got an S rank capturing it.
In general, the problem lies in the power of your styler and the length of the line. The higher they are, the less circling has to be done and the easier the capturing.