Why do I say so? Well, it's based on my experience of using different types of rosins on Erhu.
The Pirastro's rosins I've used on erhu before were the Eudoxa, Gold and Gold Flex. None of them were very suitable for Erhu steel strings if compared to the Schwarz. Furthermore, they are more expensive.
I don't want to describe technically how good it is, but at least to tell you that Schwarz rosin is specially made for steel strings, and I just feel comfortable and easy to control.
Cheers~
