Nothing I can think of... Should not be an issue. We have two Gateways, each pointing to different LB'd Storefront servers, one setup with SAML(Okta)+FAS and the other with LDAP still. The users that don't use SAML are none the wiser that they're doing it the 'hard way' w/ LDAP+RSA SecurID for MFA vs SSO/SAML and MFA Push.
The only thing holding us back is apps that still use AAA-TM SSO and don't have modern auth flows yet, that users SSO into when they provide credentials at the Gateway.