I have had nothing but good experiences with Zero's customer service over the years. But I haven't had an major problems since 2012.
In 2012, I had two minor issues with my 2012 S. One was a stalling problem caused by a defective throttle assembly and the other was the "glitch" recall. In both cases, Zero sent a truck to my home, picked up my bike, took it back to the factory, performed repairs and returned my Zero back to me a few days later. The 2012 S is owned by my daughter and is still running great, although the motor's encoder device failed, causing random stalling, but my son-in-law bought a new one from the motor manufacturer for $30, installed it himself and the bike has been running fine ever since.
My 2014 S had a defective power tank when it was first installed by my dealer. A Zero technician drove to the shop, installed a new PT, took the old PT back to the factory and then installed the latest firmware, all of which took two hours and at no cost to me. After that I never had another issue with the 2014 S, other than the battery pack is slowly loosing capacity. But my daughter still has that bike and it continues to run perfectly, also. The bike never needed to be returned to the dealer for repairs or servicing. (That was a good thing too, as the shop went out of business a few years ago, which had nothing to do with their Zero franchise.)
So far this year I have contacted Zero customer service twice regarding minor issues, which had nothing to do with the 2018 S that I just bought. Both times have received a response just a few hours later. One issue was regarding getting their 2018 lineup approved for the CA EV rebate and the other contact was because about a missing part on a Zero accessory that I had purchased through my dealer. Both issues were corrected within a few days of my contacting Zero.
So far I have had nothing but good experiences with Zero's customer service.
But of course, I am keeping my fingers crossed.