Yes true, part of the thing is about third party not providing true vst3 support. And that’s the case for waves…
But nonetheless you have to get your hands on a protools to get the multi-mono thing completely. And it’s been around for rtas an tdm since forever… Not only aax.
As far as I can tell, in nuendo, you can target whatever channel(s) you want with a (true) vst3 plugin. Completely cool.
But you simply cannot target say 5.0 channels out of a 5.1 track and in the same plug have a different states of the plugin for, let’s say LCR and LsRs.
You are forced to use two plugins.
Or else I have (hopefully ?) missed something for many years 
In that case you have to put 2 instances I the same vst3 plugin on your 5.1 track (one targeting LCR and the other one targeting LsRs)
In protools you can insert a multi-mono plugin and then you can create one group with LCR, and an other one with LsRs.
And that goes for every plugin.
Frankly, I’m quite happy with how nuendo handles it, but I can also understand people around who would like to do that but with only one plug inserted.
Have both possibilities would a plus, even if I’m not sure it’s doable technically…