Yes, I understand how anycast works (I work for an anycast based CDN). The issue is that you can't do percentage based routing with anycast... in fact, you can

Yes, I understand how anycast works (I work for an anycast based CDN). The issue is that you can't do percentage based routing with anycast... in fact, you can ONLY do shortest hop routing with anycast (at least for WAN anycast). That means that, while different edge networks can go to a different datacenter, every individual edge network will hit only a single datacenter. The key issue is that anycast is a very blunt tool. You are relying on your BGP announcements to route traffic, but you aren't actually in control of where a particular request goes.