I’ve read a few of these threads on Reddit and I’ve never seen any good arguments for the Necromorphs, aside from explicit advantages given to them over the flood.
a single flood spore in atmosphere will infect an entire planet in a small period of time. It spread over a city, from a crashed ship, in hours. It’s incredibly virulent and resourceful.
Necromorphs take a much longer period of time to warp humanity in its area, making them more and more insane, killing each other and all that.
In terms of who’s moving quicker and growing faster, the Flood will always have that advantage.
And once the flood reaches a grave mind, I think it’s over for the Necromorphs. Those things are too intelligent and organized, on an organism that’s insanely hard to put down.
As for actual combat, Master Chief can canonically flip warthogs with one hand. Flood forms can send Chief through the air. Most Necromorphs are being bullied