From what I've been told by people who know way more about it than I do, it is complete. One thing to consider is that official published precedential caselaw is from the appellate level up. From what I understand, lower court cases aren't published, though I believe they can be accessed through PACER... But I have no real legal research expertise. The first part of the project entailed a lengthy process involving several decades-experienced legal librarians, lawyers, and archivists mapping out exactly what reporters exist and which ones were "official" (considered authoratative by the courts) and when. Apparently nobody had done it before— well, nobody that made their data available, anyway. HLSL is a library of last resort for law and all but a tiny fraction of the books we scanned are in their collection.

In the about page there's more detailed information about the scope and process.

this makes sense, thank you!