If you modify a Ford truck and that modification kills you, you're liable.
If you modify a John Deer tractor and John Deer didn't do everything in their power to stop you from making an unsafe modification, they're liable.
Furthermore, if a farm worker gets killed, maimed, injured from the framer's modification, the farmer is immune from liability and it is still John Deer that would be liable.
https://nationalaglawcenter.org/workers-compensation-for-agr...
> Whether or not someone is eligible for workers’ compensation depends on their state and the industry they work in. There are no requirements at the federal level that mandate states to have workers’ compensation laws. Nevertheless, every state, except for Texas, requires most employers to carry workers’ compensation insurance. However, the majority of state workers’ compensation laws specifically exclude or limit agricultural employers from the workers’ compensation requirement.