They're paid so much because you need to pay twice for the bad ones. My house's previous plumber threaded a copper line directly into the stainless steel DHW tank. That wasn't a problem, until it was.
I’ve never encountered a DHW tank made of stainless steel with stainless steel fittings.
The error that will destroy your pipes is to use anything copper in the condensate plumbing of a condensing boiler. Use PVC, CPVC or PEX (or stainless steel) with plastic fittings (PVC, CPVC, “engineered polymer”, or John Guest ProLock or similar). The high CO2 concentrations in the condensate will rapidly corrode copper.
The error that will destroy your pipes is to use anything copper in the condensate plumbing of a condensing boiler. Use PVC, CPVC or PEX (or stainless steel) with plastic fittings (PVC, CPVC, “engineered polymer”, or John Guest ProLock or similar). The high CO2 concentrations in the condensate will rapidly corrode copper.