Biggest advantage for me to switch to OVN is not needing to run an ip namespace for l3 router services, no need to run keepalived etc. OVN runs l3 within openflow and that simplifies deployment. Also L3 failover is very fast. OVN does DHCP within openflow so that it reduces BUM chattiness on the network. It doesn't ACL in openflow so no need to run iptables etc.. There are many things OVN does and more to come in future.
Downside of OVN is troubleshooting, it requires some good knowledge of openflow and its components etc. otherwise it rocks.