To fail from pure pulling, the pin would have to sheer through or the plate around the slot would have to bend such that the pin could bend and pull through.
Even assuming the cheapest grades of steel involved, the geometries are such that the forces would need to be in the 10's of thousands of pounds. Probably higher than the chain itself is rated for.
I would be mostly worried about the spring-ring getting snagged on something or somehow bouncing loose. On tractor implements, lynch pins get lost all the time being dragged through brush and stuff. On the trailer, its probably less likely.
- Rick