The main issue is the mismatch of version numbers for ARIA between Mac  (2.039) and Windows (1.967). Files saved on a Mac with VST data cannot be loaded on Windows, as ARIA complains that the data is "too new".

I agree!

The VST3 version of ARIA for Windows is a mess.

Not only will it not support multiple audio outputs...it can't do multiple instrument channels either. Seems like there are other issues as well.

Multiple channels are imperative for those moments when we choose to use channel bouncing techniques to change instruments/articulations in a Dorico expression map.

As more hosts drop VST2 support, Windows users are finding we must resort to some kind of VST2<>VST3 bridge, or use the Plogue Sforzando player instead (and this one can't do multiple channels either...Dorico cannot 'plugin bounce' at this time either).

The point about score exchanges (Dorico/Sibelius) between Windows and Mac user is very true indeed. I'm always having to rebuild ARIA instances because of this. We cannot easily exchange playback templates and such across platforms either.

Another important reason to get this fixed ASAP...

Windows for ARM is quickly becoming a reality. VST2 is highly dependent on intel/amd instruction sets. VST3 isn't supposed to require such instruction sets, right?

I do wish Garritan would fix it, or reach out to Plogue and let them make it right.

There are a few minor things across the board with several Garritan libraries that shouldn't be all that hard to fix, yet they've gone neglected for almost a decade.

Personally, I wouldn't object to paying a reasonable fee to get a solid multitimbral sfz player like ARIA, and fixes for the libraries that people have pointed out over the years.

I came across something of interest in the Dorico forums today...

GPO5 for Windows test please - Dorico - Steinberg Forums

Looks like it's possible to get VST3 versions of ARIA working in Dorico for both Windows and Macs (including the new ones with Apple Silicon). With benwiggy's instructions, scores and playback templates are interchangeable between Windows and Mac versions!

While the Windows VST3 version doesn't do multiple audio outputs, it at least seems to support hosting instruments in the different slots assigned to different MIDI channels if we need to do some channel bouncing and such for custom tweaked layouts. That's something I was unable to do before, nor can sforzando (always been a single instrument plugin) do that.

The trick is to make sure you have the latest version (v1.959?) of the ARIA player installed from Makemusic support. Grab and install the latest version of sforzando, which will update the ARIA engine to the latest version (currently 1.978) fully supported by Plogue. While the engine with sforzando is numbered lower than that included with ARIA installs from the Makemusic support links, I believe it is newer and more up to date stuff.

benwiggy dug up some information....

Plogue have always been fairly tight-lipped, though it now seems clear from comments that they are planning a 'next-generation' player; and that they do have an eye to backwards compatibility. 

If no ARIA updates come from MM, then Plogue may provide a way to keep Garritan going.

Does the sforzando player only work at 41K?

Firstly, you don't have to use the SFZ player. The only reason for installing it is to get the ARIA Engine version up to 1.978. This is the most up-to-date version (e.g. MM's "version 2" is actually older code than this!!); and it is a version that can be used by both Macs on Apple Silicon and by Windows PCs.

The Engine is the bit that does the audio work for ARIA Player (app, VST, AU, AAX, CLAP plug-ins...) and for SFZ (and other apps, too) : everything else is just an interface wrapper. 

In short: SFZ and ARIA are essentially the same thing. I can change the sample rate in SFZ's Preferences; though this might depend on the capabilities of your audio interface. Also, the Garritan samples are all at 44.1, anyway.