My experience has been a little different.......
I once blew the engine ECM on a Gradall mine scaler while welding on the ROPS. Batterys were disconnected and my ground clamp was only inches from my welding.
Our Gradall rep told me they recommend unplugging all ECMs before welding.
A New Holland tractor had the transmission ECM cooked when a Alamo boom mower was being installed and welding was performed. Again I was told by New Holland that ECMs should be unplugged before welding.
I have also been told this by our Kobelco service rep.
I DO know once you cook a ECM it gets pricey