One of the biggest issues was deciding the ship classes for each of the ships to fall into. Obviously, the Federation ship needed to be an Escort and the Romulans had the advantage of the anything-goes Warbirds, but it wasn’t immediately obvious whether the Klingon ship should be a Raptor or a Bird-of-Prey. For more details on the design process and the cross-faction differences for all three ships, check out the full diary.
Source: Making of the pilot ships