From my experience, my R5 3600 spiked to 94c when doing 90% folding. I tend to leave 1 core untouched for PC background processing & to handle GPU folding.
The temp i got, that's on the wraith stock cooler, which i reckon is the culprit for your sudden reboot.

I also noticed that, on my current setting, the TPF (time to go up 1%) really is astounding. Less than a minute. Folding using AMD CPU is a breeze compared to Bulldozer/Phenom days.

and i believe those with R9 3990X, can perform 3-4 CPU folding, running at once. Provided the cooling is exceptional.