We have been using generateRandomSessionId  to aid our research on sendBeacon support:
And found it to be not so "random".
We have about 56.000 records in which our client side generated id should be unique, but on those there are 29 duplicates. (Please note that this is nothing to do with sendBeacon itself as these numbers come from the control experiment that is contrasting "regular" event data with "send beacon" data)
We have looked into the browsers that are produccing duplicated data and for the most part is Safari in its many flavors (we can provide more detail data upon request).
Perhaps we should consider using: http://caniuse.com/#search=getRandomValues when supported?
We have enough "good" unique data to do our experiment so that is no issue, this is just an FYI.