NFS / SMB

I use a synology NAS servr as main server since Synology supply all needed software. The NAS is linux based. I do have Windows and SLE in LAN. The server offrs SMB and NFS.
On SLE I started using NFS since I think this is native Linux where SMB is native Windows.

But are there differences in safety and functionality?
My feeling is NFS is faster access.

When I use VPN (openVPN vi SLE) I canøt access my shares via NFS. I need to use SMB mounts.
Why that?