Plus if you take close look at any of the decks out there you will find that they all have been welded at the factory. So that local engine guy is going to be wrong as I don't think the manufactures are not going to intentionally weaken a deck knowing the abuse that the decks are put through.
The biggest problem stamp steel deck repair is the burn through caused by hidden rust damage or using a too higher amperage welding mode. Sheet metal take a delicate touch and not everybody can do it.
Those engine guys probably will also probably tell you that cast iron, aluminum, and plastics can't be welded too. I have done all three; although, I haven't mastered the aluminum welding yet.