The joint is very strong. A weld destruction test will usually show the metal tearing before the joint. ICAR welding classes require the metal to tear or have pullout for plugs in order to pass. Also keep in mind this is for oem repair procedures and all vehicle manufactures have different requirements. The ICAR stuff is overkill for any restoration type panel replacement, but you can do your own destructive test with some scrap sheet metal similar to the car. Cut a few note card sized pieces weld them together and bend back and forth in a vise. That's the easiest in shop test before welding on the vehicle. Practice different sized gaps and even backers to see how it works and how far you grind it down. It's surprising how strong it is with proper gap. This is an older article but it explains the process better.

https://rts.i-car.com/crn-137.html