AH I see. When we talk about nodes in MongoDB they are the elements of a replica set. That is what confused me. Yes, you are right, to get the equivalent performance of a two node database server you would have to run a sharded cluster with two shards. That is definitely more nodes than the two node design you have outlined for the RAC like design.

I am not familiar with Oracle RAC architecture but casual mention of a SAN fails to recognise the cost and complexity of even the smallest SAN. On the other hand MongoDB can run on dumb disks without a SAN or RAID required (though RAID can help with performance).

With you two node system loss of a node results in a 50% loss in performance whereas with MongoDB loss of a node will result in a secondary taking over the primary role with no loss in performance. You get what you pay for.

Setting up such clusters used to be a PITA until the advent of MongoDB Atlas. Now you can set them up with a few mouse clicks.

As regards shard keys. Yes, they are immutable, but the documents they index are not. So dynamic schema properties are preserved in a sharded cluster.