Earlier quoted context omitted.
ACT is pretty well established scientifically at this point. There are controversies but not more than other things and they tend to be focused on the novelty of ACT concepts. As to any particular implementation, that's a different issue.
Its my own ignorance, but can you please link to data showing effectiveness?
So it's interesting. Looking at the meta-analyses more closely than I have in the past raises more questions about ACT than I thought based on the way it's discussed in the literature. However, in general, the pattern is that it shows efficacy relative to controls but not greater than other treatments (although this is a common pattern in the psychotherapy literature that applies to almost every type of treatment):
https://www.karger.com/Article/Abstract/365764
https://www.sciencedirect.com/science/article/pii/S016503271...
https://www.sciencedirect.com/science/article/pii/S000579671...