^I don't agree. I think it's likely that it's something like what Ketrick
suggests -- that those early cloaks were just some sort of holographic camouflage (consistent with the Romulan technology seen in the "Babel One" arc) or sensor masking, something which Federation science had long since penetrated, so that what we saw in BoT had to be true
invisibility, something that even the best Starfleet sensors couldn't see through -- and that's
what Spock was saying was extremely difficult to achieve.
Sure, there are times when it's just too much trouble to rationalize an inconsistency and it's simpler just to accept it as a continuity error and not worry about it. But this is not one of those times. It's not that hard at all to reconcile. Particularly since it fits neatly into the progression of different cloaking technologies that's been implicit in ST since 1968. It would be a mistake to treat all cloaking technology as a single thing. Common sense alone tells us that there would be a constant competition between stealth and detection technologies -- the idea that cloaking tech in the 24th century is anything like cloaking tech in the 23rd is irrational on the face of it, even aside from the onscreen evidence. There would have
to be a progression of different stealth technologies which might not be related to each other at all, which might use totally different methods of concealment than those used by previous stealth technologies that were penetrated decades earlier. Even if one were creating an entirely new universe and postulating the existence of some kind of cloaking tech, that would be a logical assumption to make. And it just happens to provide a perfect explanation for the apparent inconsistencies in the onscreen portrayals of cloaking tech in ST. This is one case where those onscreen inconsistencies actually make things more