Symptom:
Today, IOS XR accepts exp-null, but rejects imp-null when acting as penultimate hop for a sub-LSP

Conditions:
Facebook's using none traditional P2MP data center replication
Facebook is pushing unicast not multicast traffic into P2MP tunnel. So RPF check is not needed.
Facebook setup P2MP tunnel btw Datacenter routers. So bud node is not a concern for the FB deployment.
To enable UHP on juniper router is large change and has performance impact.  PPS performance is key for FB. 
Facebook wants Cisco ASR9K to accept the implicit null from juniper tail as the RFC does not really specify that UHP is needed,