Proof of concept, more than effective needs...
Assuming Green Lan 1 192.168.10.0/24, and Lan2 192.168.20.0/24.
Assuming than Lan2 needs only few services on LAN1, just like an FTP Server, a DB server, a Printer server and an Application Server, which are on different IP addresses on Lan1.
With NAT on OpenVPN, i could use only 1 IP address for allow access to all that services on Lan1 with port forwarding from OpenVPN adapter (for instance 192.168.12.1).
Without NAT, i should use specific firewall rules for allowing services/ports to the Ip addresses located on Lan1, and configure clients to "look for" the services and the IP addreses on Lan1.