… 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.
I’ve been using iA Writer since forever, and I would love to switch for the wysiwyg display alone. But I, too, work on Mac and iPad with iA Writer, and I find it annoying to work with different apps.
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.
One of the great things about the Apple ecosystem is that you can switch devices and use many of the same apps. With iCloud Drive, it’s almost as if Apple devices were think clients.
I write with iA Writer, and I ofter like to reread and edit my articles on my iPad, to get a different view. With Lettera, It wouldn’t be that big of a deal, but there is, as the comment says above, a cognitive cost to changing.