When my link to one of those three ISPs goes down, I have a 1/3 chance of each outbound connection failing because the return path is broken (or my other two upstreams do uRPF). I also have a 1/3 chance of each inbound connection failing until I update DNS to remove the relevant AAAA records; smarter clients will try multiple addresses for inbound connections, but there'll be a delay and not all clients are that smart.
Generally applications that support IPv6 support step-down through addresses, otherwise dual stack hosts that don't have a v6 connection can't connect to services that are dual stack. This is the kind of bug that IPv6 enabled applications get fixed extremely quickly.
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