I generally proofread articles with Grammarly; the free version, just checking for basic errors. With iA Writer, since it is bare markdown, I can paste articles in Grammarly, get corrections, and copy them back. I just tried one with Lettera. The result was a bit problematic. H2s were converted to bold text, and there were same ~ characters around a couple of links. The text wasn’t very complicated: just text, headings, and links.
Also, I just found when converting a file to HTML with Marked, that there are ~ characters around most of the formatting. I assume that’s Grammarly not interpreting the markdown from Lettera, but it’s probably the wysiwyg causing the issue.