The idea behind it really appeals to me. However, Guix is so niche that I felt like it was not worth the effort to actually daily drive it. I went the NixOS way instead and have been daily driving it now for almost 2years. I’m really satisfied with the paradigm immutable and reproducible os. I also manage my servers this way and it makes it really easy to rollback stuff.
The learning curve is the same as for any language but you have to relearn how to manage an os this way as it can be really different than a trad os. It forces you to really understand for example how packages traditionally expect to link to various libs available on your system.
I’m pretty sure their app are Foss software. Here is the android client https://github.com/ProtonMail/proton-mail-android which is GPL 3