First off, there's a LOT of different kinds of breathwork out there. They each have nuance, caveats, and some dangers associated with doing them wrong. I did dirga pranayama breathwork with breath rentention for years when I had a different schedule in the mornings, and never had the muscle thing you mentioned. The only danger was in doing it wrong.
As for holotropic - here's a study of 11,000 participants, where only 2% didn't have any sort of interesting experience.
There were no adverse reactions or unresolved negative outcomes.
So, based on this, evidence suggests you might have simply not done it right. Which is common with people doing breathwork on their own. YT videos don't often train you as well as a real human watching how you breathe.
As for calling it "dangerous" and you projecting that on accomplished "I'll try anything" author Michael Pollin - why? Lots of things are dangerous and we do them every day. Driving a car kills about 40,000 people in the US. If he mentions driving a car, do you also groan and think "Don't do it!"? Likely not because that's something mundane to you.
The need for a trip sitter seems (as are the effects of breathwork according to a lot of people, honestly. It's cool, but it's work, not like dropping acid) overblown. Psychedelic experiences last about as long as the breathwork lasts - it's that people keep up the breathwork for hours. Feelings of euphoria might last all day, but the need for a trip sitter seems unnecessary, IMO.