I'd never heard of Polarr before, but I've just downloaded it and tried it out. As a Lightroom 6 on Windows 10 user, this is an impressive little app!
A couple of questions...
1. Using the Windows version, where are saved photos stored - locally or online? If it's online, what about when you have no internet connectivity?
2. I haven't been through the legal stuff yet, but do you know if using this app gives the provider any rights to use your photographs now or in the future?
EDIT: I already answered #1 - there's a "polarr" subdirectory created in my Pictures folder