The hardest part has been to clearly define what experiments we need to run on our product for the FDA to accept it. There is nothing inherently difficult about this, but the problem is the lack of available information and feedback cycles to get the information.
For example, the FDA has a feedback process called 'pre-submissions'. This is good, but the problem is the cycle to ask a question and get an answer is 3 months.
I'm happy that there is a growing group of entrepreneurs working within the regulatory system. Hoping to see more tools soon.