These wirenuts have a VDE mark on them and are according to some German electricians legal to be used. In the Netherlands and in Italy wirenuts seem to be fairly common.
I bought them since they are a bit smaller than strip connectors of the same capacity. I don't know of any box fill restrictions, so I have to work on many old overstuffed boxes. The old twisted and taped splices were much smaller than strip connectors, and the basic rule of thumb for box fill was: you can cramp in what you can cramp in! The old boxes were often only 4x8 cm and in thin walls 2 cm deep. In Brick walls you can go as deep as you want, these "boxes" are just wooden frames without side walls or a back. A handmade sheet metal cover is then screwed or nailed onto the frame. These covers had hole patterns, the smallest size typically 2 round holes, the larger ones five holes (a rectangle with one hole in the center).
When I did our dining room I had to resort to twisting and taping since I couldn't get enough strip connectors into the box. Here the wirenuts come in handy.
