There are no safety issues involved in intersection take off's.
Before V1 you stop. After V1 you go. The calculated V1 speed will take the shortened runway into account. Extra runway is of no use to you as you are going to continue with the take off after V1. You are not going to ellect to stop after V1 because you think you might have some tar left. That would in fact be highly unproffesional.
Intersection take off's where correctly applied save time, brake wear, brake overheating(taxiing for 21R full length in JNB).
It's important to remember that you are not always field length limited. You can also be 2nd Seg climb limited, brake energy limited, Structural Weight limited.
All the finest airlines in the world use this option to provide flexibility to there operation.
Once again. There are no safety issues involved in intersection take off's if you understand the principles involved.