As someone who has years of experience using 185s, I can safely say failing air con is not a new issue at all. They're probably second behind the 158s if I'm honest. It's never aided by traincrew insisting on leaving saloon end doors open to try and circulate air across carriages, which simply makes the aircon on the neighbouring vehicle work even harder to minimal effect on the next carriage.
I'd also add they're about as unpleasant as a train can get when the aircon does fail. Above you is the beating sun, below you an extremely warm engine, in the middle are some very sweaty passengers.
Conversely, I've yet to see an aircon unit fail on a 195.
I think, ultimately, you're asking a lot for a diesel engine to drive an aircon unit of the size required. EMUs seem to have issues far less often, I suspect due to the readily available, consistent supply of electricity.