Well, as an aspiring software developer, I'm gonna say that if it is a software that you install on your phone/laptop and neither the recording nor the processed data is stored anywhere else, nor shared with anyone else, I'd be okay with that part of it. Of course if you need to upload it to a server hosted somewhere else, this is a big no no. The company to date says it is deleting the recording but keeping the transcription for the purpose of researching effective therapy: Security Policy - Mentalyc
I would not be okay with that, but if a patient is, I can see how that might be valuable information as a basis specifically for research purposes.

That being said I agree with LT that I think there is value in the therapist taking notes.