The idiotic thing about high-current appliances is........ controlling them from some printed circuit board.
Printed circuit boards are just that.... printed "wiring", with components, relays, etc., connected via paper thin copper "traces".
Even if printed traces are made wide enough, they are still paper-thin, and prone to overheating if subjected to high currents passing through them.
They certainly are not comparable to say, a 12 gauge copper wire.
The same thing can happen to relay contacts, with the additional pitting of contacts due to the spark generated when they open up.
Over time, this then causes the contacts to not securely make a solid connection, adding to overheating.
Components handling high currents, like relays, are soldered to those traces, and solder is also prone to failure, because overheating happens, softening the solder.
And those spade lug connectors are a poor choice, particularly if they are made of thin metal, and not gripping tightly.
All the above issues contribute to increasing resistances to the flow of electricity - thus overheating and failures.
You've got heavy amperage to control, ten, twenty amps, etc., and to reply on such a vunerable string of components is a crap shoot, indeed a "weak spot" in the reliability department.
My one air conditioner used cheap chinese spade lugs to connect the compressor, and failed after years of handling the high current.
I took care of that crap and went to tough, tight, quality spade lugs.