Wordstar certainly had a multiple-document mode by 5.x, but my understanding was that being written mostly or entirely in assembler (the configuration utility would patch the binary!) vs. however WP was developed allowed WP to outmaneuver it. More likely (and particularly on-topic) is that Wordstar “.DOC”s were relatively human-readable save for some odd choice of CRLF character… so it was easy enough for WordPerfect shops to suck in others’ work, but the WordPerfect format was more opaque as far as getting text back out unless you started using the actual software.

OpenOffice/LibreOffice has not been entirely bug-immune within its own format; in particular it’s been surprisingly easy to get into nightmares trying to insert text before or after a table (or a Table-of-Contents/Index object) if you didn’t pad it with some empty paragraphs and plan ahead, since UI corner cases will always be corner cases. The extra decade or so of legacy support in the Word format(s) does make it easier to end up with a genuinely “haunted” document, though.

Here in the legal salt-mines, much of the overpriced templating stuff that would otherwise run fine in WINE relies on OLE or similar bindings to an actual copy of Word to generate/render output (although the last one I actually tried to use was old enough to be equally satisfied by an actual copy of WordPerfect for Windows).

RTF: The joy of RTF is when you and probably a lot of holdout WordPerfect users spent some well-meaning effort badgering the local (government-employed/contracted) court reporters to use it instead of Word .DOC back in the mid-2000s… only to find out that the page numbering is generally done with an automatic token and the layout is only guaranteed to paginate correctly in the exact version of Word the RTF was generated with. Of course, the alternative is PDF, but at least 8GB RAM is now cheap.