… I will, for now, stay with iA writer. The reason is pretty simple. I also edit MD text on iOS, and I don’t like to work with different apps on iPad OS, macOS, and iOS.
But otherwise, Lettera is very stable in the first tests I made. Solid experience, and it’s a very clever move to bring that app.
We humans are a weird species 
Instead of enjoying the one thing in that one place we have, we want it everywhere, or prefer to suffer everywhere all the time at once!
It’s not that simple. Every app has its own logic, conventions, keyboard shortcuts etc. Switching between apps has a real cognitive cost.