On a whim, I've been trying out posting to this blog from iA Writer. It sorta-kinda works, but gives me a singularly unhelpful error message even though the posting worked. It also dumps the draft post as a HTML blob, which can break in odd ways if converted to blocks in the Gutenberg editor depending on how I'd formatted things. (I was using some old stuff I wrote on Tumblr back in 2016 as test material.)
I think I'll stick to writing my posts directly into the Gutenberg editor on the website, or else copy-pasting from iA Writer.
(Yes, I'm aware that Ulysses has improved their WordPress game, before you comment — however, I'd want to double-check that, and to be honest I'm quite happy with iA Writer at this point and would need a lot of persuasion to pony up for another subscription.)