Well... I ONLY use IllustStudio, so in case you were referring to my Tumblr, yes, everything is IS... 99% ,1%could be some SAI.
Bear in mind, it's only a tool, you can perfectly imitate traditional media in PS, but process will be different. All of my "traditional" brushes are custom, EVERY SINGLE one, carefully adjusted to my stroke and pressure, you won't find them in basic brush pack, nor CLIP. It is really up to you to work around and see what does the trick.
I can only advise this software, from my personal appreciation, for I find it perfect.
But in all honesty for 70$...
What are you hesitating about lol?
And thanks guys, I am flattered! :3
One of the tricks is really: Stick to only 1 layer, pretend it is a sheet of paper and work as if with subtractive color.