It appears it's just the one item
It looks like the only function that's not available is the ability to upgrade HALO software and "observe controller failover." Perhaps Nimbus hasn't worked out how to seamlessly failover through an upgrade on an active system. I'm not familiar with Nimbus/HALO, but it's going to be an operation that's got to execute carefully. If Nimbus' configuratin is active/active, you'd have to first force all operations over to one controller, then upgrade the other, then force operations (these are live, active I/O controller sessions) over to the newly updated controller then upgrade the second controller, then go back to splitting operations between the two controllers.
Our NetApps do this OK, but I'll admit to being impressed by it generally working without much drama. Oracle can't support it with many of its database patches, even in a RAC - you have to shut down the database to patch. (admittedly disk I/O is much less complex than database transactions).