One of the challenges of being a good SM is what they don't know because when someone states mater-of-factly that the SM can't be a MBC, the SM might believe it, thus tying their hands of an opportunity for helping a scout’s growth.
I believe the biggest barrier for scouts’ growth in all units are adults creating rules, regulations, restrictions and protocols based from their fears. I used to teach a class on this very subject helping adult leaders get their program past the adult fears. Look at the post in this discussion and look at the self-created MBC “guidelines”. Most if not all are based simply from the fear of an adult making an unethical decision. National doesn’t support these guidelines, but you would think they were written in stone by the tone of scouters giving them.
The OPs basic question was “Do you know if there are any limitations or restrictions for the merit badge process when a SM is also a MBC?”. He followed with reasons for asking the question, but the simple answer is “The limitations for a SM are the same as all other MBCs”. Now some adults' fears may lead to adding more requirements and that is fine because all of us do it to some degree. But when we add requirements, we risk creating a barrier to a scouts growth because we lose the meaning of the process or method. From Nationals perspective, there are no more requirements on a SM being a MBC than any other registered scouter. And as far as I can tell, the MBC is the highest authority of the scout's performance for recognition of MB requirements even if the SM has a place to sign after all is done. Seems silly to me personally because I can't imagine how that signature could trump the MBC, but it's that fear thing again.