Road salt , that and that alone.
Down here in the depths of SW Ireland due to the Gulf Stream running along the coast winters were historically very mild .That changed two years ago but even then there was and still is NO network for spreading salt on the roads and grit, as in stone grit, is used instead , indeed many locals have never seen actual road salt.
There is an abundance of Co. Kerry registered cars from the early 90's running around with little or no rust on them apart from being inflicted by a myriad of bumps and scrapes from the " unique " local driving attitude and ability.
I've never heard of a car being "put down " due to welding being required except if it was a UK import.
Vans are a classic example . Most vans on the road from the early noughties were UK imports due them being so comparitivly cheap and only requiring a 50 euro registration fee. Nearly every Transit and Vivaro brought over is riddled with rust whereas there are some locally purchased vans , some going back to the late eighties , which are in remarkable condition even though they were no better cared for than the UK sourced ones.
When I think back to the UK the copious amounts of salt that was put down each winter which was then turned into a strong saline solution that was in turn sprayed all over a car running down into the door cavities etc. and up ito all the corners underneath it's inevitable that cars will rust so badly.