Stereo monitor cueing has a few beneficial advantages, however it certainly is not universally better than mono cueing!

1) In or over the ear direct monitoring of a stereo mix is the most efficient delivery of the desired “separation”.
2) Some performers prefer BU tracks in one ear and their contribution in the other.
3) Stereo mixing for speaker stacks and wedge monitoring is “Fools Gold”.

Most all of my 50+ year career has been with acoustic Americana music performance and this is what I know. It is imperative for performers to be able to hear each others contribution and understand the critical difference between BU rhythm vs a lead role. The best performance possible is when the ensemble gets a wedge cue very close to the house speaker stack distribution and can manage their own dynamic control. Hot back lines and todays “more me” musicians require custom individual cue mixes that absolutely places the dynamic control of the performance at the console and not with the musicians on stage: it usually does not deliver to the seats a better performance for acoustic ensembles.
Large scale sound reinforcement pretty much requires console control of everything. However in most mid to small venues delivering a clean mono distribution with stage wedges providing the portion of the performance that is absent with “the stage bass bloom” from the speaker stacks is the most efficient protocol for artist control of their performance. IMO direct in or over the ear distribution of a stereo mix is as good as stereo can be: from that point on the further from your ear that stereo mix is is sourced, the less effective it will be!
Hugh