Iv'e got the following explanation from NetApp support. Hope this helps people who've got the same issue.
ONTAP 8.x with RHEL 6 client.
Configuring sec=sys on the client and sec=none on the filer, with NFSv3, the client makes a "AUTH_UNIX" call and the filer responds with "AUTH_NULL".
However, this is not the response that the client would expect. Therefore we reply incorrectly with only 1 flavor (we should be replying with 2), because our code does not allow for "AUTH_NULL".
There are a number of internal cases open within NetApp, but there is no expected fix as NFSv4 will take care of the issues encountered with NFSv3.
So the advice from NetApp would be to use, sec=sys on the client, sec=none on the filer, using NFSv4 from RHEL6 and upwards.