Alternative point of view: it's not about boundaries but about rules and control.
My mum would tell you she had strict boundaries with ne growing up. I would tell you it was her way or the highway, and she got so mad about stupid, inconsequential things that really didn't matter, that there was nowhere left for her to go on the big things that did matter.
I remember one of the worst things she did was a punishment because I forgot to go to one half-hour long music lesson which was usually after school but had been moved to lunchtime one week because the teacher had another appointment. I lied and told her I had gone because I knew she would go completely mental and massively overreact, and if there was a chance of her not finding out, I would take it. Of course she did find out, and the punishment was severe.
Did I learn that my mum had strict boundaries that I was not to cross?
No. I learned that she had absolutely no sense of proportion and a complete inability to control her temper.
From that moment on, whenever she got angry, I assumed that it was probably just because she was crazy and not because I had done anything objectively bad. I obeyed her most of the time because I was living under her roof and she was able to control me through things like access to money and lifts (we lived somewhere with little to no public transport). It didn't make me have any more respect for her though.
Now the boot is on the other foot somewhat, in the sense that I am an adult with my own life and my own money, living far away, and she wants to make sure that she gets to see me and my kids, which involves a long journey and a lot of money each time, so she's a lot more careful about falling out with me. I could use this to my advantage and be really controlling of her and her behaviour, but I don't. Because I remember both how it felt and how ineffective it was.
So yes, by all means, have boundaries in place with the people in your life, particularly the people who need boundaries because they don't just behave properly without needing to be told. But bear in mind that if you fly off the handle about cake one week and then the next week your mother flouts your no social media rule by posting a picture of your child naked in the bath to Facebook, how do you communicate the relative severity of these offences when you've already gone straight to level 10 over the cake thing?
You can't.
And then the person will no doubt go around telling anyone who will listen that you are horrible and controlling and you have rules about EVERYTHING, and because they will bring up that time you went mad about cake, everyone will sympathise with them. Whereas if you accepted the cake with a polite smile and a thank you, but went nuclear over the social media thing, there's a much greater chance that the person concerned would take you seriously, or that anyone they told about it would side with you and say, "Actually Linda, she is right. You shouldn't put pictures of children on Facebook if their parents have asked you not to."