Your risk #2 is an extremly valid point - the demographics of toolbar users are not representitive of internet users in general (for instance there is no version of the toolbar available to the 10% of us who prefer linux or mac and associated broswers). Alexas top 100 [alexa.com] makes it painfully obvious that their stats favor those who will install anything - amoung the top 20 are several susspect sites including for instance, gator (aka claria, a spyware advertiser) and an ip address that hosts no content. The last thing the internet needs is spyware number one for say "desktop weather" or "wallet plugin" or anything remotly similar.
That said the thing that could differentiate google from alexa is factoring quality of traffic rather then quantity only. When the top alexa site has 16 page views on average and the top spyware has 1.8 (both stats gathered from alexa), your able to quickly gauge which one is providing a service and which one a disservice.