We deployed 300 Z3s and 40 MX64s back in April, 250 of which were to migrate a contact centre to teleworkers. They all have Avaya IP physical handsets so use the PoE on the Z3 for power and a desktop that was literally picked up from their desk at work and taken home - the z3s tunnel back to a pair of MX250's in our datacenters.
To date, we've had no call quality or app performance issues - the only problem being a frequent tunnel drop (every 20 mins or so) which was caused by an upstream firewall in the datacenter terminating the sessions unexpectedly.
Future considerations for us is NAC - as we had to deploy in less than a few days, we had no time to provision acceptable physical security to prevent the user plugging something else into the Z3 and getting to the corporate network - this is on our roadmap.
if you want any further info, let me know - it's a fantastically simple deployment and once the groundwork was completed we didn't even see the users or their devices (inc the Z3s) - they just took everything home and followed a plug in guide.