My first thought is that it might be hard to prevent decay and leaks into the structures supporting these spongy surfaces. 80cm of water logged soil above a parking garage, for example.
The renderings they are showing for newer plans (at 1:27 of the video) seem to indicate a soil later running a long the wall of the building, in addition to the roof. As the result water would naturally travel down that wall and into the surrounding soil, preventing the buildup on the roof and negating need for a traditional drain.
That said, what would be required to make sure no barrier formed, say from the roots system or something else is really hard to say.
You'd also need some sort of lining that would prevent the moisture from being obsorbed by the building materials themselves.
Planting grass on top of parking garages isn't that unusual or new in Germany (though usually done for aesthetic reasons), so I assume this is a solved problem.
Sufficiently deep buildings and parking garages might also encounter ground water, so adding soil on top might not even add new problems to the engineering side.
Similarly, the rooftop plants are shown on rooftop designs that would already have to deal with stagnating water
That said, what would be required to make sure no barrier formed, say from the roots system or something else is really hard to say.
You'd also need some sort of lining that would prevent the moisture from being obsorbed by the building materials themselves.