To be clear, I understand that VST 3 plugins should support multichannel audio automatically. However, we don't live in this perfect world quite yet - mono and stereo VST 3 plugins remain a reality, as does the problem I describe here:
Protools, Sonar, and other DAWs offer a convenient mechanism for employing mono or stereo plugins in a multichannel track. For example, Sonar's SoundBridge automatically instantiates multiple copies of a mono or stereo plugin in the background, as necessary, to affect all channels of a multichannel track. Furthermore - and this is the important part - all plugin instances are linked so that adjustments made to the first instance (or any instance, for that matter) are automatically replicated to the others. In my experience, this is intuitive (even invisible to the user) and usually works well.
As it stands, Cubase 9 supports the ability to launch multiple instances of a plugin in a multichannel track and to route audio as necessary to affect all channels concurrently. It's not automatic and it's not particularly convenient, but it works. However, no facility exists to link those plugins within the same track. [To be clear, Cubase supports linking of multiple plugin instances when those plugins are located in separate tracks (and positioned at the same insert level).] Consequently, employing mono or stereo plugins in a multichannel track is virtually unworkable.
Take, for example, the common application of inserting your favorite stereo compressor plugin on a multichannel drum buss. Even if you're only working with the LCR channels, it's very difficult to dial in the ideal compression settings by ear when forced to jump back-and-forth between two instances of the compressor - the first affecting L/R channels, and the second affecting the C channel. A minimum solution would be to allow users to link the two plugin instances together so that when the threshold parameter, for example, is adjusted on the first plugin, those changes are automatically replicated on the other and can be heard instantly on all channels, thus making the process of fine-tuning the parameter in the mix straightforward, as when using a purpose-built or VST 3-compliant multichannel plugin.
Now, getting back to the subject of VST 3 compliance, I'll sorta understand if Steinberg blames plugin developers for this problem (though it's been discussed for years without resolution) . But it's also understandable if developers don't rush to retool their plugins when the audience for surround tools is so small and when other DAWs offer such convenient workarounds, as described above. So, who will fix this? I suspect a long wait unless Steinberg comes to the rescue. [Please, Steinberg, come to the rescue.]
To conclude, at a minimum, please allow for the linking of identical plugins in a single track. Or, if you're feeling generous, add a convenient, purpose-built utility similar to Sonar's SoundBridge.
Thank you for your consideration.