[bunje]

I would guess that it is much more dangerous to modify a modern car because they utilize nontrivial shell geometry to improve the rigidity or to reduce weight. When I was attending driving lessons, my teacher claimed that the specific shape of the windscreen or rear window was a very important safety feature in the scenario that the car rolls over.

[sgt101]

The big one is the A pillar on the front. This tends to corrode over time and is often why cars are written off. If it is corroded then the structural integrity of the car is compromised. There is really no repair for it - but a bit of welding can disguise things and get a write off sold as a potential project. The new owner can find out when they take the wings off... or they can find out when the front of the car comes off in the event of a shunt, but in that scenario they only find out very briefly.

[jacquesm]

That write-off is an economic affair, not a technical one. A-pillars too can be re-manufactured, but it is usually better not to because if the A-pillar is compromised you are looking at a body that will have a whole lot of other damage as well. The bad part about damage there is that you need to build it up from the inside out which means 'undressing' the whole box layer by layer so you can rebuild it properly, and most people will not have the time, the skills (or the jigs) to do this properly. But it definitely can be done, but likely not in a way that is economical.

I can't imagine an A-pillar that is damaged with the rest of the car being deformed as well, the A-pillar of a modern car is ridiculously strong and meant to be the outer boundary of the safety cage for the passengers. Any deformation there and you'll have a deformed bottom, roof, firewall and probably other parts as well.

[TravelPiglet]

Not to mention crumple zones and how the car is intended to protect the people inside

[jacquesm]

Those are typically outside of the safety cell.