The third party tracking was dependent on someone else's processing power and system, this is sinisterly making the user's own software work against his best interest and instead work for a morbid advertiser's interest. Since it is all done in the browser and locally using much more of my data, that data being even more 'relevant' to advertisers, with its results presented as an API to every bloody page I visit, this will only make the tracking problem worse.

A related reason is my bookmarks are very much a private affair and this method feels much more intrusive. I dread to think of the security implications.

The only upside is that third party cookies will be less of a problem in other browsers, if this new API kills them.