Any FireFox Send security concerns in the community?

Looking for comments on the overall security of FireFox Send (send.firefox.com).  My understanding is that all of the security tasks regarding the file (encryption, link generation, decryption) take place on the client side.  However, also received some sage advice that what a provider 'says' and what they 'do' (federal FISA warrant) can be two different things.  

I've done some basic searching online; have found some tutorials and an explanation by Mozilla on how the system works.  Really looking for some independent, third party vetting results.  I have looked on Git Hub, but didn't see anything that conclusively stated FF Send meets standard xxxxx or is xyz compliant, etc.

Is there any documentation out there verifying Mozilla's security claims and does anybody have any concerns with using this product when sharing PII (e.g. name, address, health status, income, etc)?