Nith,
I'm sorry, I don't thing so. Render function is called before the form is displayed on the screen and it's function is to preset workflow fields with actual process, task id, submit URL, server email and so. At the end it injects javascript called form bridge to the form. I checked forms that were submitted well, forms that cannot be submitted at all and forms that was submitted after some time of clicking the button. I have not found differences, all the forms where prefilled well.
The submit function is called after the form is submitted and data are passed to the workspace-server. That data are input parameters of the submit function and you can validate them, make computations and so on. Unfortunatelly in my case this sometime never happen. Form is unable to submit the data. And if data are not submitted, no submit function is called.
For that reasons I do think there is no problem in render / submit function. 
--- Jaroslav