- mycroft intends to launch openSTT
- and you can use a local Kaldi server already for offline voice recog (haven tried it but push request is in github)
- and there is several options to choose from where the voice is processed, you can always add more as they appear (choose who to trust, revoke trust without changing product)
you can just proxy your mycroft connect to the server , make a new pariign code that didnt ever run on original connection, and you will be using a shared api even on googleSTT, they have your voiceprint sure, but less data to process it since its mixed with lots of user’s requests
and amazon is big, for profit, they WILL try and harvest more data for sure,
making money out of you and listening to your voice requests is a recipe for …
also, do you know when it is recording? is it open source? or does it just promisse to not record on demand by amazon?
mycroft CANT spy on you, you know the code, it listens when you order it only, guaranteed!
privacy by design, not by policy, thats how it helps me in my privacy concerns, hope this answer helped you making a choice 
EDIT: i actually want to discuss this subject better, so i created a topic Privacy Concerns and Discussion