RDMA can be rather challenging for a cloud provider, as it typically requires a lossless fabric. Cloud providers don't want to enable flow control on switches (to prevent packet drops) as that has the potential to deadlock the entire network if a buggy or malicious VM stops consuming traffic.
Heck, I took our corp. net down a decade back by causing a test machine to drop into a kernel debugger with flow control enabled on the switches and the test machine. All of a sudden, my remote access to the serial console kernel debugger froze, and I was cursing the flaky VPN. Then a second later, I had the 'Oh Crap, that was me!' realization.
Loss is inevitable in most cloud network fabrics due to the use of shallow buffer commodity ASICs, the amount of fan-in, and the reliance on tenants to use the network in a typical manner (don't fragment, don't encap, preserve entropy).
RDMA is going to be tough - microseconds count, you can't wait for TCP to retrans, and UDP doesn't provide the reliability needed. A new transport protocol is likely the answer.
Heck, I took our corp. net down a decade back by causing a test machine to drop into a kernel debugger with flow control enabled on the switches and the test machine. All of a sudden, my remote access to the serial console kernel debugger froze, and I was cursing the flaky VPN. Then a second later, I had the 'Oh Crap, that was me!' realization.