I had a nasty experience that cost me 4.5 hours with 12.14. The existing network was already running 12.14. A switch (an MS225) was newly added to the network.
The switch showed as being online, but failed to take any config changes (the time in the dashboard would keep incrementing). On the local status page it reported that communication to the Meraki Dashboard on tcp/7430 was being blocked. The thing is - this was an additional switch being added to an existing network that already had many Meraki switches.
Regardless, I spent much time going through the firewall, taking packet captures, etc. Tried factory reset.
Eventually called support. Actually I tried many times. But the DTMF tones were not working, and I could not key in the customer number or case number, and there is no way to speak to a human without doing this.
Eventually, I had the idea to make an international phone call and try support in another country - which worked. So it was just the support phone number in my country that was broken.
Support found the version of firmware shipped on the new switch was unable to upgrade directly to 12.14, and the switch had gone into an infinite upgrade loop which was preventing it from taking new config (but allowed it to show as online in the dashboard). The switch was communicating fine on tcp/7430.
Support had to pin the switch to an interim firmware version and let it upgrade to that, and then remove the pin to allow it to do the normal upgrade to get it working properly.