Most of the candidates I take through the FI course do not fly very well, nor do they have good technical knowledge.
Particular weak points are:
Flying an an accurate and well managed circuit
Flying and managing a PFL
Steep precision turns
Cordinated turns
VFR navigation
Dealing with abnormals and emergencies.
The amount of technical knowledge you need to be a flying instructor is more than you need to pass a professional pilots exam in the subject areas that are relevant to the PPL.
I have to spend valuable time on the course re teaching candidates to fly rather than teaching them to instruct, Candidates need more directed learning while hour building. All a 100 hours of straight and level teaches you is that light aircraft are quite stable.