I think it depends a bit on what you’re doing with your other training, particularly running. My N=1 with triathlon training is that too much running intensity quickly leads to niggles and then injuries. So I build my running mileage by running frequently (5-6 days a week) and mostly at fairly easy intensities. Most of my cycling sessions then include some intensity, so my overall training is quite polarised, but I’m not doing many low intensity workouts on the bike. I also found from experience that I don’t get a lot of benefit from lower intensity cycling unless I go really long (4-5 hours plus). If I’m just training for cycling then I’ll do lower intensity rides for recovery between hard days, but with tri training I find that time is more productively spent running or swimming.