Well, it doesn't work. Not sure I'd call that a risk though :-)

How did that even come up? Why not use an ampersand instead of a forward slash? Ampersands have a well-defined meaning in the query part of the URL, while slashes don't.

Personally, I would favor the URL shortener. It is a useful feature on it's own, good for branding (if you don't shorten, many sites will shorten for you using their own schema, which results in nondescript URLs), you get nice URLs (in the short URL you can just factor the parameters into the shortened part, in the full URL you don't need them because the user has been counted already), you get less cache fragmentation (even if you remove the parameter in Varnish, you'll still fragment the client cache). On the negative side, it's one more request so clicking through becomes somewhat slower.