I found this article to be one of the more complete and objective that I’ve read comparing iA Writer to Ulysses. It is a little dated (2017) but the fundamentals are covered making the article relevant today.
I also found this article by @MereCivilian helpful.
Finally, this article provides an overview of several writing apps that I found useful.
After striving to eliminate a subscription and to find “a writing app to rule them all” I gave up after realizing that I was making things unnecessarily complicated. I’ve come to realize that I am the most productive with two writing apps, each with distinct advantages for the two types of writing I do. Specifically, Scrivener is best for my book projects because it has robust organization and research features, e.g., a built in outliner. Ulysses is best for my short form writing such as blog articles, work communications and presentation notes. Accordingly, I’m using both and I’m quite happy with this approach.
I’m at peace with me decision. Now, back to that book project …