vagfilm It looks simpler and more efficient than putting a pair of magnets with opposite polarity to "speed up" the engaging of the jack (I think it is Steingraeber that has this in their top line of uprights).
That is correct, and it has the advantage that the magnets are easier to manufacture uniformly from one to the other and will not change their properties with time and use, unlike springs. So they are "better"
CyberGene Many people say they prefer tall uprights such as K500 and U3 to baby grands in terms of sound but the grand piano action is better.
Many people say that (including myself 😁), but in reality not many are really able to play at a level that makes a difference. What is really "better" in the grand piano action is that it relies very little on springs and a lot on gravity. The reverse is true for the upright action: gravity plays no factor, but springs do, even more with this mod. Fact is, it is very difficult to have uniformity in spring behavior across all notes, so the action cannot be very accurate. This is usually not a problem, because the action being wooden (see below) has other source of inaccuracies and so it does not matter too much. But many people who tried the WNG composite grand action describe it as "so incredibly accurate and predictable" that it'd allow a superior level of playing (I haven't tried one myself)
CyberGene I’m wondering why systems like that are not more popular.
The grand piano has the benefit of being much nicer looking (arguably) and of allowing the performer to look around to the public and/or other performers. This is the true reason why the grand pianos are "better" and more popular.
Moreover, the whole piano industry is driven by very weird things. For example many manufacturers claim a huge superiority of the "wooden action", which is clearly false. There might be arguably some benefits of keysticks being wooden, but composite actions are undoubtedly superior. Yet, they are very unpopular and the only "major" brand utilizing them is Kawai, which has a very small fraction of the world's market -- and they get unfairly bashed by people for having "cheaped out" the piano with "plastic".