As of recently, there is a strong correlation between use of em-dashes and a verbose, bloated style of writing that uses many words to not quite get to multiple repeated points. Not well written, in other words.

The combination of both I usually find in text generated for me by LLM, and so far, it serves as a good signal for density of information and, inversely, not-quite-right ideas and their connections. (That's not to say it's pervasive, but below the surface, the quality of LLM text as a medium is often not great.)

There may be well-written text of non-trivial length generated by LLMs, but so far I haven't knowingly encountered any, with or without em-dashes. Human editing remains very necessary.