When it comes to stress tests, FurMark is one tool that does not always receive the best rep. Given this, many gamers choose to avoid it completely as a precaution.
While FurMark is not inherently bad, unlike other GPU stress tools, it does tend to add extra stress on the GPU that is not always necessary. While some GPUs will be able to cope with this, others may not be able to.
Given this, FurMark may potentially cause the graphics card to overheat, which can cause irreparable damage to the GPU. If you do want to use FurMark, it is a tool that you should use only for a very short period, rather than for a few hours. Definitely a god GPU stress and stability test.