Also, it's true that a lot of the existing tools that do similar things are anything but free.
I can imagine most or all of the things ArkhamMirror does are done elsewhere by other programs and tools. I don't know of any unclassified projects that do ACH better, but that's a pretty niche tool, and the government loves their 20-year-old software solutions.
Off-the-shelf programs designed for use by lawyers have layers of protections built in to make sure they are suitable for court-use. I don't make any claims as to the legal utilities of this program whatsoever. In fact, the ACH PDF report generated specifically calls attention to the AI-generated nature of the materials and warns against using any data generated or entered without human review and approval.
That said, you can make some pretty cool, non-legally useful, connections with tools like author unmask, where you feed the system docs by a known author and run them against docs written by an unknown or suspected alias to check for similar voice. During ingestion, the system automatically yanks all detected Regex data and puts it into a nice sortable, searchable list for you.
Legal e-discovery products are going to be highly polished, reliable programs designed to be used in a legal setting, while ArkhamMirror is designed to be used while you sit in your faraday cage in your hacker cabin in the woods with no Wi-Fi.
No shade intended - my stuff's not nearly as pretty or as well-put together as a decent off-the-shelf e-discovery program and I'm not trying to imply that it's better in any way, it's just differently aligned.