I'm well-educated like many here, but not a medical expert. I can tell you what I have consisently heard and read over the years
1. that I have never heard any recommendation that it is ok to take cascarada sagrada or senna continuously long term. and 2. that I have always heard it said and read on labels and articles that these are considered more along the same category as stimulants and laxatives and should not be used longterm which may create dependence.
Of course this answer does not satisfy your request to be supplied with the once and and for all facts....Be well