Personally, I find zone 3 sort of a happy spot. In zone 3, I ride faster, and although the effort feels sort of hard, it is not as awful as zone 4 and above and not much different from zone 2. And I get to my goal distance faster or turn around point faster. Zone 2 seems more boring.
Many times consensus data is very accurate. I am sure
RChung can comment on this.
By consensus (BF consensus), I mean that to build mitochondria (genesis) one has to ride long hours. This favors zone 2 riding. Zone 4 and above generate more mitochondria and mitochondria enzymes and turns on genes but generates more fatigue. Zone 3 is over too fast i.e. the mitochondria meter does not run long enough.
Conclusion: Long z2 rides build the number of mitochondria and zone 4 and above bulks them up. Zone 3 is a meh and may over a long period of time cause one to get slower - too much time in a happy spot causes malaise, genes turn off and mitochondria reduce their numbers: think plateau. And one could just ride zone 4 and above and metabolically do great and generate and bulk up mitochondria. Alas, one generates too much fatigue, burns a lot of muscle glycogen per hour and does not get enough saddle time to support one's weight and posture on the bike for any substantial goal distance.