It is hard to find facts about it. When I did research all I saw was people using it. It may be more opinion then scientific fact right now. It makes sense, yet like you, many have used it without any ill effect. I would do what has worked best for you, and if you find something negative, fact wise, then you can take it into consideration.
There are a few different different ways to care for bloating/constipation in any fish, so to me it's a matter of what is the healthiest way. Since bettas are insectivores, I would go with the frozen daphnia, then Epsom Salt since it's very gentle and safe, pea being my personal last go-to for the reasons I stated above.
Why I go with the wording "recommend" because it may not be idea in ways, it does tend to usually work.
I used to be strict on telling people not to, and sometimes I still do depending on the situation at hand.. but now hours after researching it and with such little evidence on the negatives it's hard to say absolutely one way or another.