What are the limitations if any of the form calculation feature

I'm not sure what is the scope of the limitation, but if you are doing a calculation and the math function you are using is available in our end, it should still work. 

You can check this guide on how to do a form calculation: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form.

For the list of Math Functions that is available, you can refer to this link: http://www.jotform.com/help/267-Form-Calculation-Math-Function-Reference 

If you could provide us more details of how your formula or form works, that would help us better assist you.