If you're implying that either of those are the end all, beat all stability test, you couldn't be more wrong. There are no stability tests that cover everything (especially IBT and Furmark), and no amount of testing will guarantee complete stability. Which is why using multiple different tests, that test on a wider variety of instruction sets, it's ALWAYS the best option. Configurable testing platforms like AIDA64 and OCCT for heavy load testing, and Realbench for lighter load testing (some will even argue that it can be used for heavy load testing, because it does include AVX instrucitons), are all superior, and won't JUST create an ungodly amount of heat for a LINPACK based test. Granted, both AIDA64 AND OCCT both use LINPACK, but also have other instruction sets, and are also kept updated with new AVX instruction sets. Something IBT hasn't done....as it hasn't been updated since like 2012.
For GPU stability, I've found the best combination to be Superposition on 1080 extreme, Firestrike Ultra (graphics test 1 and 2 on loop), and Time Spy Extreme (graphics test 1 and 2 on loop)...each for at least an hour. If there's instability in your overclock, that'll very likely find it, without submitting your GPU to such harsh environs that furmark offers. The only thing furmark is good for, is cooking eggs...and GPUs. Any additional testing should be done in the game you're playing, or the hashing program that you're using...because as I stated before, there are NO stability testing programs that will catch everything. None.
The thing is....unless you're building a rig for a very specific purpose, like data mining or video editing, you aren't going to be pushing your rig as hard as ANY of these stress tests will. Most people are just gaming, and doing light compute work, so Realbench for a few hours is MORE than enough. But if you're data mining, video editing or doing anything else that requires precision and stability, overclocking is probably the worst thing you could possibly do. I have a rig built specifically for data mining using both CPU and GPU compute, and I know better than to overclock it....because there is one simple truth to overclocking stability, and that is, that there is NO test that will guarantee it.