As many people said the CFI can be difficult. It also depends how much prep you do before you go. My school made me have intructor outlines and lesson plans for each item in the each of the Private, Commercial, Multi, and CFI PTS's done. The school ride was difficult, but it helped me to pass my all of my CFI rides the first time.
My intial instructor ride was the MEI, which means one thing, you're going to teach VMC. I had awesome handout to teach with, and that impressed the D.E.. When I went for the CFI and CFII, I was asked all sorts of creative questions. The examiner's thinking was, since I already had my Initial Instructor, I knew my stuff, but he just wanted to see how I could apply that knowledge.
The instument wasn't too bad for me. If you have simulators to practice on, it helps. I did my instument in a Seminole, so that adds some extra factors in the ride such as engine failures, etc.
As with what was said before for the commercial, the 180 degree poweroff landings are tough. There are very tough margins for the landing, and with winds it makes it even harder to judge. Even though you can safely land the airplane on the runway, it has to be within the PTS limits. I landed 100ft long of the PTS limit and failed my ride because of it. In my opinion, that is one of the hardest part's of the commercial ride.
Tim
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PHX
