It is yet another shame that ENT Minefield needlestly showed a Rom ship cloaking even though the dialogue from TOS BoT made it quite clear that it was a newly presented ability for them. Spock even talked about it as theoretical - not something that had been observed in action a hundred years before!
And Spock was dead wrong there. He himself had witnessed a ship appearing from nowhere in a previous adventure, "Charlie X": trying to claim that some sort of a theoretical invisibility device is the likeliest explanation for the same happening again makes no sense.
Invisibility of the "surprise the enemy" sort is quite unlikely to ever have been anything to write home about: any enemy worth the bandanna should have that ability, be it due to stealth, speed or jamming, and Starfleet should have standing orders regarding such things. And even bona fide failure to cast a shadow is highly unlikely to come as a surprise to somebody in the 2260s, really: we soon learn that invisibility is commonplace and humdrum, and there's no reason to think this would be a new development. It always was commonplace and humdrum in the Trek universe, regardless of whether humans stumbled on it or not. And Kirk being the first to stumble on it is implausible, whilst "Minefield" is plausible (and only comes after "Unexpected" anyway - Archer saw an invisible ship right off the bat, just as he should!).
(Now, DSC repeats the very same thing in "Vulcan Hello", with the heroes agonizing that the Klingon ability to appear from nowhere is new, surprising and inexplicable. But both "Vulcan Hello" and "Balance of Terror" are at fault in the exact same manner, making general, overarching observations about the capabilities of a mere specific culture. In both cases it would suffice that the ability of Villain X to go invisible is surprising; in both, the ability
an sich is.)
The Romulans were also stated to not have warp drive, so how they managed to fight an interstellar war, including reaching the Sol System, is kinda baffling.
Nobody ever says the Romulans lacked warp drive.
The Romulan
ship the heroes met was claimed to lack warp drive, which is a completely different thing (I can't fly; I'm a man, and man can fly). And the claim was probably dead wrong anyway, with Scotty being fooled by this submarine-like vessel that switches from diesels to electrics as the tactical situation warrants. Just a few episodes later, Romulan ships of this exact same shape perform warp intercepts with ease and without comment.
What we learn from Insurrection is that humans feel warp drive transformed Romulans from thugs to an empire. We never learn when this might have happened. But there's no indication Earth was fighting thugs in the 22nd century war, and indeed "Balance of Terror" makes a pretty direct reference to them being the Romulan Star Empire ever since the old war (that is, we see a map with this very name writ large for the enemy territory, despite the fact that nobody has interacted with the Romulans after that old war).
Timo Saloniemi