I've seen the warnings of having UFs sending HFs and relaying to a index cluster. The objection seems to be around uneven distribution of events from the HFs to the indexers. However, doesn't setting the HFs outputs.conf variables: indexAndForward=false and autoLBFrequency=30 fix those issues?
Basically, have the UFs load balance to a pool of HFs and the pool of HFs load balance to the index cluster. If you do this, will you still have problems?