So what is interesting, and it may be obvious that TrainerDay is a predominantly a European user base. We have may US folks such as @Jeremy but still mostly Europeans. Spain is our biggest country and Europe is probably 10X the US in the number of daily visits.
So I am listening a lot to Dr Stephen Sieler lately, the great sports scientist from Texas that moved to Norway 23+ years ago… So he felt that his schooling in the US led him to similar training philosophy as Coggan. This educational bias came from all the research in the US being done on untrained individuals. PS: You can’t compare me to Seiler, but we have a few things in common in that we are both American’s living in Europe, and I now realize I fully support his suggestions.
After deeper research in Norway on TRAINED athletes, Dr Seiler’s opinion changed. He started presenting the idea behind Polarized training, and even the European scientists did not seem overly open to the idea at first, although many top European coaches said yes, this is how we do it.
So this belief has a heavy distribution towards Zone 1 in a 3-Zone system their studies show it produces better results in athletes doing as little as 4-8 hours a week. This seems to be the standard in elite athletes doing significantly more hours. My European 30-year pro-coach friend says he does not focus on this distribution, but his data aligns with this and is his recommendation for pros and non-pros alike. He suggests for beginners the distribution should be even more Z1 focused. Z1 is your aerobic threshold. Sieler definines this as the place where you can do hours or riding with no HR decoupling, below the point where you start to feel your breadth or talking is not super comformable.
Here is a good podcast where Dr Sieler talks about this and some history behind this.
Dr Seiler also feels the sweet-spot focus originates from Coggan’s book “Training with Power,” because in-theory, this is the zone that “tick’s all the boxes.” I would call this science theory vs evidence based science. I don’t want to put down sweet-spot training as it produces fast riders too.
My consusion is that following a polarized model is likely to be more effective in many individuals. You can keep it super simple. You only ride one hard interval session every 5-workouts (1 of 5 = 80/20) or you can do time in zone but that should be closer to 90/10. Dr Sieler also suggests that he believes (not sure if he has done studies) but that it’s better to focus your HARD WORKOUTS on threshold or slightly above and keep increasing the duration. I don’t like these intervals 
I should state that I know this is not American vs European perse, but it seems to be weighted this direction.
