Hi and welcome,

As it is an Elixir forum, You might have strong opinion for Elixir, but here are some good/bad parts for Elixir.

• Phoenix is very similar to Rails, and this might be easy to switch
• Elixir syntax is Ruby like
• Elixir really fits chat like application

On the bad part

• Elixir is not Ruby, and the learning curve might be higher, because You need to switch from OOP to Functional programming
• There are probably more packages availale in the Ruby world than in Elixir, although I could replicate my RoR dev to Elixir/Phoenix

Also there is no Active Record, but Ecto… no callbacks, but multies.

What’sApp was built with Erlang, so You see You might have all functionalities available for languages running on the BEAM

Mixing 2 technologies for the sake of productivity might be possible, but in the long term, You should stick to one framework… it adds some complexity when trying to communicate between both applications.

You can have a look at a chat application in Elixir here.