My reply isn't technical or anything but I can share my experience. My OEM charger on my 2016 Zero SR with 23k miles just recently failed as well. I happened to live near the official Zero dealership in Orange, CA. I was going to do a direct battery replacement until I asked the owner of the dealership for his recommendation. He said that the OEM Calex charger has been improved and there has been less failures, but it's still an old design that Zero won't probably spend money on fixing. The charger on the new SR/F is a new design by Zero and they probably worked out the kinks.
With that said, he advised me to buy the Quiq charger for a near zero-fail charging solution. He said they are solid, never seen one fail, the cooling/connections/contacts are beefy and should never experience heat issues. So I went with tge $600 Quiq charger since I just use the bike to go to work. The other solution is the $2,300 charge tank option but that's for someone that likes to ride long distances often.