Clearly not all dealers are bad, I'm sure most are honest and have integrity, but like most any industry, there are some bad players. All it takes is a few bad guys to make a lot of people unhappy, and here is a great place to vent, so we tend to read about it often. Sad to say I have personally witnessed scams perpetrated by a few of these bad actors. Either way, my advise of getting a second opinion and asking for bad parts back will not affect an honest dealer, but it will help weed out the bad ones.
Also with damage like that there's no way the that car would even make "ready". In a case like dwg's where the car was still drivable there's no way it could be a catastrophic failure like that. So yeah I'm also really suspicious of this ECU replacement, and if it came back completely non-functional (as in not even being able to make "ready" whereas before it could do) then I'd be even more suspicious.