Is it stable now to overclock?
Game was always stable to overclock. It depends entirely on how strongly and in what manner the program pushes it.
Witcher 3 just pushes your GPU harder in a different area (If I were to guess, I would put my money on shaders in W3) than any of your other games.
Every program has a different stable max O/C on any given type of silicone, be it RAM, CPU, GPU or whatever. The best example I can think of is with a Haswell CPU, I can run the XTU stress test for 24 hours straight at 4.8 ghz on my 5930K, but using Realbench to stress that same overclock I get Blue Screen of Death 10 minutes in and only get it to run stable 24 hours straight at 4.6 ghz.
This does not mean that Realbench is a bad program for stability testing a CPU. What it actually means is that XTU does a poor job of stability testing by not pushing it hard enough. Same principle here with W3 and GPU's. It's not that W3 is a bad overclocking program to test GPU stability, it means it does a better job of it than the other games do.
As a side note I can run Heaven 4.0, Valley 1.0 and Firestrike Ultra tests all day long at 1540 mhz on my GPU's, but in order to stay stable during Witcher 3 I have to drop my O/C down to 1495 mhz. I would hardly call 1495 mhz on a GPU that runs stock at 1000 mhz (boost to 1180 mhz) "being unable to overclock"
If you want to run the same overclock on W3 as in your other games you are going to have to add more voltage to your GPU or add some better cooling to them. If you are at max voltage and using a Peltier cooling set up running around -100 C* on your core, then you sir have found your max overclock for your GPU in W3.