Some forms may have a high frequency of completion. For example US withholding statements may have to be completed on a monthly basis. Korean beneficial ownership forms may have to be submitted with relative changes in CIV participants.
Complexity of the content
Some forms may require interpretation of complex tax rules from users that are not trained in the legislation of the tax form.
Some forms may require supporting documentation that is not readily available. For example, some countries require transaction reports to prove that a minimum holding period was met.
Lots of countries have certain unwritten requirements that you just have to know from experience. Meeting these requirements significantly increases chances of successful recovery.
The completion of forms in an incorrect way can be severely penalized. A form W-8BEN-E is for instance explicitly signed under penalties of perjury.