I worked in legal tech/AI. It's relatively easy to reach the capabilities of Legora and Harvey. My finding is that it's the industry itself that's very tricky and old fashioned. Lot of legal contracts at the large law firms (which Legora/Harvey are targeting) are controlled by iManage and NetDocuments systems. To integrate into their workflows you have to be "chosen" as a development partner. The said law firms also care very deeply about security and audit trails, where the documents reside, how they are processed, etc - at least on the surface. Despite their size and wealth they have invested very little in their in-house data & IT capabilities - maybe they have couple of techies working full time, but mostly they outsource to other companies. They also still print documents and do manual redline. And on top of that you have this insane GTM from Legora/Harvey where their sales peeps are crazy good, and work insane hours.
So it's not really a technical challenge, it's more of an industry challenge, so they will happily buy Legora/Harvey licenses. But maybe (probably) this will change over time.