It doesn't really surprise me that the Romulans would stop messing with the Klingons and go under cloak when they saw the Federation ship approaching. OTOH, if the idea was just to destructively raid the colony, I'd think four (Type-A? C.f. "The Defector") warbirds would already have made short work of the Klingons and would have no reason to loiter. Perhaps the Romulans were searching for something there? Or perhaps the resistance was fiercer than we were let to think?
It's possible that the E-C went in not knowing what was happening, and indeed had shields down. What we hear is this:
"We were responding to a distress call from the Klingon outpost on Narendra Three. The Romulans were attacking it. We engaged them, but there were four warbirds."
Quite possibly the distress call didn't tell what was happening, and the "Romulans were attacking" bit is something Garrett only found out after arrival. And evidently the distress call didn't reach anybody else besides the E-C, which sort of supports the idea that it might have been weak, partial, jammed, whatever.
I like how the impulse engine of the ship goes from the "systematic" red to the "canonical" blue due to battle damage... OTOH, it's dissatisfactory visually and dramatically that the E-C isn't firing any torpedoes in the first half of the battle. When the E-C meets the E-D, her "photon banks are depleted", which sort of suggests that ammo was used up, rather than that the launchers failed when still full (although we later hear that the launchers did fail, too).
Overall, it looks a bit unrealistic that the E-C could survive against those four ships for any length of time when the ship types seem evenly matched (althought the E-C does demonstrate some of that supposedly superior maneuverability). It would have been a more "sustainable" fight, dramatically speaking, if the E-C had eliminated one of the opponents early in the game, perhaps thanks to a piecemeal Romulan attack (and the attack we saw here was indeed piecemeal - two ships joined in later). Tasha does speculate that there would still be four ships facing the E-C, but what does she know?
And agreed that the "anomaly appearing ahead of the ship"/"ship spending several seconds sailing in" isn't a good match of dialogue, nor visually all that interesting. "A bright light, and then here" could look spectacular from the outside while not leaving the people inside any time to react.
Timo Saloniemi