I don't like because it looks like an organic, giant, dark hole. Like a scar from a former growth.
To me it looks like it breaks the flow of the external design and yes, it does look like a massive hole that just... doesn't seem to have a purpose.
Perhaps it was a necessity for SF's first Dauntless class design? Maybe an exhaust of some kind... but I find that doubtful since SF ships as a whole don't have a huge need for exhaust ports (take Warp and Impulse for example - they don't emit any classical exhaust byproducts as such apart from some particle wakes which are associated with generation of low level/high level subspace fields - which also allows a ship to be manuevered and moved in a given direction - so both are essentially field generation and manipulation technologies).
The only piece of tech which requires exhaust in a 'classical sense' and is associated with propulsiton that I can see on SF ships would be thrusters.
Sure, plasma and other things can be vented into space, but generally it isn't because a SF ship is fully self-enclosed and recycles EVERYTHING.
Most exhaust ports on a SF ship are there in case of emergencies (aka, overloads and the plasma or other things NEED to be vented out into space if the internal mechanism which frequently uses/recycles it is damaged and there's a buildup - this is also corroborated by how SF ships work on-screen).
At any rate, I agree that the huge hole on the back of the saucer just breaks the flow of the design... but much like there being Quantum Slipstream Version 1 (which maxed out at 300 Ly's per hour) and Version 2 (10 000 Ly's per minute and has a whole bunch of other tech that made it faster).... this is probably just a Version 1 of the starfleet built Dauntless design .. perhaps it will turn out to be a 'feature' of some kind.
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