In an interview with Lex Fridman, John Carmack said that in retrospect, Quake was too ambitious in terms of development time, as it both introduced network play and a fully polygonal 3D engine written in assembly. So it would have been better to split the work in two and publish a "Network Doom" first and then build on that with a polygonal Quake.

Which seems to imply that the network stack was about as difficult to implement as the new 3D engine.

And then you had Romero saying that Quake wasn't ambitious enough...