I plan to experiment with firmware securities. What security mechanisms are there that prohibit me from plugging in my personal firmware. I do understand some devices might be powerful in terms of security, but technically speaking what is there to stop me from putting in my own firmware? (Only thing I can think of is, the compatibility of the firmware with the hardware)
What else should I worried or bothered about before developing my own version or reverse engineering one firmware?
Edit: I want to put my own firmware in a smart watch. What security mechanisms is there that is stopping me to do it?