Yooz uses Microsoft to host its Service through its Azure offering. The location of Customer data is selected by Yooz: in this case, your data is stored in a data center in the US, with a back-up site in the US as well in order to have a Business Recovery Plan.
Your data stored at Azure through Yooz is encrypted when not in use: this adds an additional level of confidentiality at the technical level.
Contractually, Microsoft Inc. can access the data stored on its US servers, in particular to maintain infrastructures and deploy security patches. This access to data is however protected and governed in application of the Privacy Shield. To our knowledge, Microsoft has never communicated customer data to the US administration in this context.
Note that many companies in US and internationally use Microsoft hosting services on a daily basis. Microsoft makes confidentiality its priority and explains it on the Trust Center site, which publicly shares the various actions, roles and responsibilities of Microsoft in terms of confidentiality, compliance and security vis-à-vis customer data.