Seeing "Unification" brought up reminded me about "Relics". Not so much the Scotty arc, which is...
Ah, also the Cytherians from "The Nth Degree". A significantly advanced race we simply never hear from again.
Nor their tech knowledge. I mean, come on, didn't the holodeck and the Enterprise computer have a keystroke logger? If Barclay could build a spacefold drive using just what was available on the Enterprise, then it really wasn't that much beyond their current tech.
Even if they were afraid of not being able to use the tech safely, wouldn't the next logical step be build a few test ships?
You know, kinda like the Discovery and the spore drive.
Heck, Voyager has the Sulu era Excelsior logs, but doesn't have the computer data for Reg's fold drive?