Honestly I have perhaps been fortunate and have not had to interact with dealers or vendors for support in a number of years.
My opinion however is service is not what it used to be in most cases. The trend is for things to last for a period of time then replace when needed.
Best case is to have someone local who specializes in electronics repair who knows what they are doing, but even that is becoming scarcer over time.
Last hifi repair I had done successfully was with Bel Canto about 10 years back where they did a good job of replacing a defective circuit board in an amp I had bought here used. So there is that.
I also had a local dealer repair a few vintage pieces around same time ago. That was hit or miss. One repair on a NAD receiver lasted a few months and that was that.
I have also always had good experiences with getting parts for repair from Ohm speakers. Nothing I know of would make me expect otherwise these days. Their customer service has always been rated top notch by customers.
Having said that, its wise to manage your hifi investments wisely and accordingly. I will never spend a fortune for a premium priced product that I know cannot offer support when needed.