Really? There's a great deal of conflicting info on the web about this. Many complaints from folks who upgraded and can't run it. Any thoughts on why it worked for you? I've been reluctant to move to Mojave from High Sierra as a result.
What's certainly clear is that CS5 won't run on the next MacOS, which will drop support for 32 bit apps. IND CS5 is 32 bit, I believe, which is why I'm exploring Affinity Publisher.
For simple documents, the PDF import seems to work quite well for IND docs that were exported using the Adobe PDF presets (e.g. high quality print). Not so good for complicated documents, though certainly much better than nothing. I have a 62 page ebook: here's a sample page. On some pages the headers were recognized; on others, like the attached screen shot, it merged the header and the part of the first column of text.
Because the header was merged with the initial text, first column right margins weren't preserved. New text wanted to flow even with the right margin of the header, not the column. Text chaining wasn't preserved either, though it's pretty easy to restore. For minor edits, this is good enough to fake it. Not so good if I wanted to do a major revision to a section where pagination needed to change.
From my perspective, importing from an IDML file is much less useful than INDD. I guess I could go through and try to save many of my legacy documents in IDML before moving on. But my concern is what to do if I needed a document after I've made the jump to a later MacOS, and can't load Indesign to create the IDML file.