Well, various jet and rocket engines were tested in WWII aboard very large and complex bombers, and proved to be complete duds.
It's just that the bombers existed before the engines were added. This may also be the case with the Excelsior
and her transwarp drive.
Perhaps testing the drive aboard smaller ships or cheaply built rigs was out of the question, just like scramjets today cannot be tested in wind tunnels or on cheapo platforms, but must ride on state-of-the-art rockets launched from huge carrier aircraft as part of hugely expensive and complex test arrangements - and almost invariably fail, disproving the very concept that was supposed to make that particular design workable.