Basically being stable is regardless of the load. If you overheat in any of the Prime95 you are not stable.I'm sure some encoding loads can hit it hard... but try that P95 custom setup with AVX boxes ticked and see if it's not hitting your CPU hard. And the thing is, it's consistently and unrelentingly hard, never slacking off as scenes change or the end of an encoding comes up and it loops. It's just unrealistic which why I feel it's good only for derating for processing stability.
The other one is probably better for heat output though; with larger FFT's and no AVX I think it's consistently putting more of the CPU in use and that ticks up the heat output somewhat.
You can stress test in lighter SSE loads but you can't state you are stable.