Usually I have very little trouble with this check ride. Applicants usually have some teaching and instrument experience under their belt and do pretty well. But (there's always a "but" isn't there) when the applicant has little experience in these areas, we'll start to see isues.
The first issue. The CFI-Instrument ride is a teaching ride. That means the applicant must demonstrate teaching the correct information. Examples: Today's ground lesson is holding. Teach me how you want me to enter the hold over XYZ VOR and fly it with a northwest wind. Or. Explain the effect of a blocked static system on the flight instruments, How to handle the situation and the issues with using the alternate static source. Or. Teach me how to use the GPS to fly this DME arc. You get the idea.
The problem with CFII applicants with little real instrument experience shows up with a lack of detailed IFR knowledge. For example the applicant may know the 1,2,3 rule but not know an alternate is required when filing to a VFR airport. Or not knowing that the 600x2 or 800x2 only apply when there isn't a triangle "A" in the IAP note box. Inoperative approach components and RNAV IAP are another weak areas.
Finally. During the flight the CFII applicant must be able to teach and fly to Instrument PTS requirements. That's not to say the ride has to be perfect but if and when errors do occur the CFII should point them out and simultaneously correct for and explain the correction technique.