Wow funny you should bring this up LR
Expanding on the dangerous aspects of NRE I think it is a silent or not so silent killer of relationships in the poly mono dynamic.
The struggling partner is getting very mixed or opposite signals. This leads to a negative cycle of events. New partner is all fun. Old ( established) partner is moody, sad, emotional....or over the top trying to be fun and exciting, unnatural for them. Time spent or dates end up being an obligation, less than pleasant, fights and disagreements occur because of the tension and as a result neither party feels more connected in fact much less. Which sets off the repeating of the cycle or a new negative cycle.
The upside for the poly partner is they have a fun alternative. Mono parnter is stuck with a partner they feel less for ...and a partner who at the moment might feel much less towards them from a NRE stand point and a sad, depressed, needy pathetic crazy person standpoint Lose lose. Hard to unwind.