No, they are not.
I just checked Seasonic and Corsair, since both offer SFX and ATX PSUs and compared cable lengths.
Few of my findings;
Cable - SFX - ATX
24-pin - 300/350mm - 610mm
4-pin EPS - 400mm - 650mm
6/8-pin PCI-E - 500mm - 750mm
SATA/MOLEX power cables are same length between Seasonic SFX and ATX units. But on Corsair, they are shorter on SFX unit.
Just wondering won't do it. You actually have to try it out.
Viable? Not very viable but without any other option (which i find hard to believe), they may do. But i won't suggest it.
Safe? Well, if you go with cheapo extensions, which may melt when loads of power passes through them, well, do you think melting power extensions are safe?
Issues? Sure, you'll increase your ripple, for the very least. And the worst, well, when cheap extension melts and catches fire, it may burn down your entire home (yourself included if you didn't get out in time).
Only extensions, that i'd be willing to consider, if there really isn't any other way (e.g no good quality ATX PSUs sold in your area), would only be CableMod power extensions,
link:
https://cablemod.com/product-category/cable-extensions/basic-extension-kits/
Are they expensive? Yes. But then again, you'll get highest quality and reliability with those. At least to me, my whole PC or my home or even my life, is far more valuable than the 50 bucks that good quality extension kit costs. And you get to choose color theme as well (or go with solid black cables).
--
I have ATX PSUs in use with my PCs and also CableMod sleeved power cables as well, with all 3 of my PCs: Skylake, Haswell and AMD (full specs with pics in my sig).
Now, prior to purchasing CableMod custom sleeved power cables to my PSUs, i contacted Seasonic (since i have Seasonic PSUs in use), and asked directly from them, if CableMod cables are safe to use. Seasonic rep replied to me, that CableMod uses the same gauge and high quality cables as does Seasonic with their own stock power cables. So, with that, i went and bought full power cable kits for all of my PCs, to match the color themes (red for Skylake, blue for Haswell and green for AMD).