Maybe you haven't been in many situations where they have been tested, Merchant?
I am good at avoiding bullshitters, piss takers and hanger ons now. Because of my experience with emotional abuse/controlling relationships I can recognise it and I just don't go there.
Perhaps it is not those specific things, but for me, my experience of actually having to uphold boundaries and say "Actually, no, I'm not going to do that/it's not OK with me if you do that" does generally involve - not necessarily upsetting people, because of course most people are emotionally healthy and are perfectly happy to listen to this kind of thing, but the potential to upset somebody.
One example comes to mind - when DS1 was about 3 or 4, he was invited to a nursery friend's house after nursery one day. I went too, since he was little, and while the children were playing in the garden, the mum took me to one side and said "My partner smokes weed sometimes, is that a problem for you?" I was taken aback because I didn't expect to be asked that, but I didn't want to upset her, so I said "Oh, no, it's fine" but then - as they proceeded to light up in the garden right next to me and very close to where the children were playing - spent the whole rest of the playdate feeling anxious and upset and trying to think of an excuse to leave ASAP without being rude or it being obvious why I was leaving. I did not manage to leave for another hour and a half because she offered to cook for the children which I accepted.
My motivation - not to upset her or make her feel judged - overrode my protective instinct for my child and also my personal boundary. I didn't feel it was safe or appropriate to have my child playing near weed smoke, but I felt that to object would have somehow been worse. I can see, objectively, that it wouldn't have been, but it must have felt that way in the moment or I would have just said "Actually, could you not?"
That's why I say I'm poor at boundaries. Although I think I might be better now than I was then, I think I would still struggle if caught off guard like that to say no.