I couldn't say if it's balanced or not.

I do think that you're probably thinking of only certain situations: dungeon crawls, by a single-classed Ranger, without the presence of significant support. In those circumstances, this is fine.

But what about when it's used for urban assassination? That's the obvious use to me. What if it's used by a rogue with a single ranger level? What other magical effects can be combo'd with it?

I would say, considering that it's (presumably?) using normal ammunition, that it would be good to specify some mechanism of action, to guide decisions about interactions. For example, maybe the ammunition becomes ethereal? Or does the ammunition do some kind of stone-meld thing?