For me it’s not really about which method will best increase FTP. Sure I want a higher FTP - everyone does. But I think for some people, there are better reasons to explore a more polarized method.
Firstly, it’s probably less taxing and stressful on the body and mind than lots and lots of Sweet-Spot and THR training. SS generally requires full devotion and leaves not much room for other training like strength and running, IMO.
Second, not everyone is racing and really needs the ability to perform at high % of FTP. I bet many riders have never even seen their calculated FTP power for longer durations over 20-30 minutes. Many riders are training for long endurance events, or just want to be fitter on epic rides, where higher power at LT1 is more important.
Thirdly, many might have hit a plateau with SS and hope that POL will build a bigger aerobic base to push that FTP from the bottom up.
For my personal example, my recent ramp test put me at 260 watts. I HIGHLY doubt I can do that for 1 hour. I’ve been cycling “seriously” for about 3 years and this year has been my best numbers so far. My season best 60 min power was @220 watts, on a 60 min climb pretty much going as hard as I could. My season best 20 min power was 264 watts. But my 5 min power @330 is quite a bit higher on the curve respectively, so I assume I have a fairly well trained anaerobic system and this probably contributes to higher ramp test results.
Why am I mentioning this? I think this is a direct result of lots of sweet spot and threshold training, as well as the typical Strava segment-chasing and group ride smash-fests that cause me to spend lot’s of time in that “tempo - sweet spot - THR” range - or in Dr. Seilers POL model “moderate intensity”. But I feel that I’m not really building a strong aerobic base. I think sweet spot only takes you so far. I’ve seen this in my own riding, especially on rides over 3 hours. Just for some fun, go and look at your 2-2.5 hour power on solo rides or where drafting doesn’t have much effect. If you compare that power number to any zone chart where does that number fall? If it’s on the lower end of endurance or moderate intensity, around 50-60% of FTP, you might have room for improvement in your LT1. If your numbers are up in the 70-80% of FTP then you’re doing pretty good.
At least this is the way I’ve been looking at things lately.