To my knowledge the SSL Axiom do not have MADI as standard. It does have a proprietary high channel count interface similar to MADI but not compatible. It is named “Hiway”. Hiway can transport 95 channels in the same coaxial cable at 48 KHz. Hiway cables are similar compared to MADI ones (BNC connectors) but they do have an uncommon impedance (120 ohms if i remember correctly).
So the AES solution is certainly the simpler solution. I did connect an Axiom MT to a Protools system 15 years ago. This was a 64 channel link using 32 AES I/O. It was working flawlessly.
So this should works with Nuendo the same.
Do not forget to sync your Nuendo with a house word clock so that you don’t need to use the sample rate converters on the Axiom (better sound). You can use a Syncstation for this (if the studio gives you only a black burst video reference) or simply the RME hardware word clock input.
If you choose the Syncstation road, then you will be able to slave Nuendo to the Axiom MT, using a RS422 9 pin link + video ref so that you will be able to use the Axiom transport.
You’ll need something like RME AES interfaces to get the job done, Protools interfaces to my knowledge are proprietary so they will never works with Nuendo.
If the studio where you will be recording do have MADI available, then it would be simpler to use a RME MADI interface.
Be careful, the AXIOM and AXIOM MT are 44.1 / 48 KHz only devices. The sample rate converters do only accept 25 - 55 KHz signals and outputs can be set only to 32, 44.1 or 48 KHz.