Feedback regulation is more complicated in a boost converter (as compared to a buck converter). The stepped-up voltage is less directly related to what happens in the first section of the converter.
Say you want to step up 9V to 100 V. You do not have 100 V available for a feedback reference V. Therefore you must ensure the converter is oscillating on its own, independent of feedback.
You need to be sure your load is light enough so that it will permit output voltage to reach 100V.
You must start the converter operating at a duty cycle that is guaranteed to create an output V greater than 100V, for your load.
Then when it reaches 100V, the feedback mechanism must take effect, and taper down the duty cycle so output does not rise higher than 100V.
These factors make more effort necessary, in order to regulate a boost converter.