HIIT or Tabata must not be done day-to-day, everyday. With no recovery days, there will be no gains. A 21 year old is able to recover like crazy, a good 9 hours of sleep might be enough to mostly recover. For others it would not work well and would lead to over-training. In overtraining, the 100% efforts will be 90% efforts, impact the immune system, generally just not good all round.
For disproportionate benefits, one needs to define which benefits exactly. A max effort will burn a lot of calories quite quickly and potentially increase V02 max (which is highly correlated to overall longevity). Zone 2 training has become popular and has other benefits, notably increases 'fat max' threshold - which gives different benefits (specifically the ability to work harder for longer while still using fat as an energy source for the exercise).
> I found that it's indeed difficult to strike a legitimate 100% effort, even when it's only for 20 seconds and only 5-10 times.
This is essentially the point. At the end of Tabata, the last interval should be the last bit of energy you have in the tank. It should be entirely draining. Doing this routine daily will not allow recovery to then properly do the training well.
FWIW, I heard it paraphrased as this: the body has essentially too modes, hard & easy. When going hard, it only matters how hard you go, not how long. When going easy, it only matters how long you go for, not how hard. At the same time, zone 2 training and HIIT/Tabata are not mutually exclusive in their benefits, but it's more which systems receive the most benefit while other systems in the body receive benefits but to a lesser degree.