I think I have found a satisfactory answer (for me) from ISC directly (didn't seem to be able to find this easily on google though!)
Do not combine authoritative and recursive nameserver functions --
have each function performed by separate server sets. This advice
primarily concerns separation of public-facing authoritative services
from internal client-facing recursive services - administrators may,
for convenience, choose to serve some internal-only zones
authoritatively from their recursive servers, having determined that
the benefit outweighs any risks associated with this policy.
If you share recursive and authoritative functions in the one server,
then if there is a problem that impacts authoritative servers only -
for example, that causes all of your authoritative servers to fail -
it will break your recursive service too.
I'll leave this here in case it helps anyone else, or if there is anything else others want to add:)