Could always run multiple load balancers and use DNS load balancing which confounds DDoS attacks pretty well. Anycast can get weird as the global topology shifts around. e.g. a client in a threshold area may start a connection to one location and might have to finish in another. An edge case to be sure, but not impossible. Anycast would require genuine statelessness to work effectively which is doable with with some good planning around session AAA.