I run a product development consulting agency that works with entrepreneurs and early-stage companies.
For the past 8 years, we have mainly used Elixir. It is good for building straightforward web apps and APIs and great for more tricky things like chat.
We also use Ruby on Rails, mostly for existing projects.
A lot of the initial work of building MVPs does not need to be fancy, it's just CRUD. Traditional frameworks are great for that. We also use React where it makes sense. The JavaScript world has a lot of accidental complexity that slows you down and doesn't add much value. Tools like Phoenix LiveView are great for adding fancy UI without overhead.
For mobile, products need a first-class user experience, and getting access to native platform functionality can be critical for some apps. Because of this, we have historically done native development. React Native is the first framework that is really competitive, but it still has its weaknesses. React Native may allow you to share code between platforms and use web people for native work, but it can result in somewhat generic apps. If you follow the "full stack teams, not developers" approach, then speed and cost of development of native is fine. After the initial development phase, the bulk of development work ends up being on the web.