Are there any security issues in creating and using a table in the underlying Drupal database

Dear Drupal Community

I am new to Drupal and would like to store information on members logging in to my website. I am aware of the Profile Module but the interface requires some pretty controls which I have already written in php and html. Are there any security issues I should be aware of in creating and using a table directly in the underlying Drupal database - then just referring to it from my php code?

Thanks in advance
Rob