Hi Sevilla,
I have just had a look at the Hamlet website and found a picture of their German spindle gouge. I am intrigued that they have labelled this as a 'German' spindle gouge, as it looks to be a spindle gouge with a fingernail or long grind profile, which most turners now use.
Regarding use, the swept back sides allow the tool to get into finer detail work, but that does need the tool to be 'rolled' on to its side. You say it becomes unstable when you do that. Not being able to watch your technique, all I can say is you need to keep the bevel in contact with the cut, no matter the orientation of the tool, and that means you have to swing the handle at the same time as rolling it.
Hope that helps,