[jeshin]

these are with a low voltage high current winding, instead of the standard 2000v winding, though, which is much safer. The danger of the unmodified microwave transformer isn't really so much that it has exposed mains voltage (although obviously you should be careful around that, too), but that it has 2000v+ and it's galvanically isolated, meaning no GFCI/RCD will save you (and obviously the 2000v will also do a much quicker job at passing lethal current through your heart/other organs)

Comparatively, a spot welder is "just" as dangerous as any other mains-based experiment. Not safe, but you have a pretty good shot at surviving, even if you get zapped.

[mywittyname]

I'm getting flashbacks of UnderDunn's home made battery spot welder built with a microwave transformer.

[jeshin]

It's a really common project for them. Certainly a better option than a jacobs ladder or fractal wood burning, since you have to just take the usual mains voltage precautions, and not worry about arcs and other high voltage schenanigans