Klingons don't actually fight under cloak, except in TUC. Even in DSC, their ability to frustrate Starfleet depended on multi-ship action, with some ships cloaking and others decloaking in turn. And that worked fine even when the motion trackers could give approximate positions. It was deadly in DS9 where the sensors no longer could do that.
In "Errand", I gather the Klingons would have decloaked in a fairly small and weak vessel, and those don't expect to survive, so they wouldn't re-cloak, either. But Kirk would aimlessly flay at a merely previously invisible enemy since taking aim takes time...
But yes, ambushes of other sorts are possible as well, and the Gorn probably didn't cloak (or else they would have, instead of being a visible if blurry dot between their volleys). Stealth devices that achieve partial results might still be used by various enemies. And all of those that don't fool the Mk I Eyeball in the process would go unnoticed by the audience. Perhaps sensor stealth is relatively common, but so seldom extends to the visual spectrum that the Klingon system in "The Vulcan Hello" still confuses the hell out of our heroes?
Timo Saloniemi