I've been looking at what little information is available about the more esoteric features of the Stadium and it looks as though if you want enough outputs to do packing playback seriously you will probably need the expansion box.
The following will be important for me:
1. The ability the playback 4 channels of synchronised audio and send them to separate outputs.
2. The ability to synchronise Preset, Snapshot and parameter changes to the audio playback.
3. The ability to send MIDI program change, CC messages, and MTC/MIDI clock to external devices synchronised with the audio.
4. How quickly it loads a new "song" - ideally less than 5 seconds and is linked to Preset selection and Setlist order. So selecting a Preset also loads the corresponding audio and MIDI for playback, and the set order can be quickly changed using the Setlist function.
5. That it has sufficient memory to hold 4 channels of 24 bit audio for at least 1 hour of playback.
6. How easy it is to load all this information into the device from whatever DAW it has been created on. In the past I had a system that used standard MIDI files to control racks of synths and samplers as well as the guitar bass and vocal effects processors in order for my band to play live. While the playback worked really well, getting the information from the DAW into a format that the MIDI file player could use was massively convoluted, and once it had been done required me to run the "live" system to check that everything had transferred as I had intended. Also it then limited us to 16 channels of MIDI control rather than the potential 128 channels that a DAW plus MIDI interface could support. Ease of transfer (or not) could be the deal-breaker.