That may well be, I found no post complaining on Siyah.
I just guess most people also swap ROM, maybe reinstall apps (for me too much work, risk of losing some apps config or payed apps (?)), or they tweak/overclock so much that stability isn't to be expected anyway.
After swapping kernel I only did two "mount --bind" (that gave my S2 a new "life lease"

), but later I stopped executing them at startup hoping that that might be the cause, but it was not.
I also installed stock 4.2 Camera app. After swapping the kernel "android.process.media" started hogging assaults on my CPU (too many photos? some of them photosphere?), so I executed my new root powers and treated it with "kill -9" a few times.
So I agree, the suspicion to Siyah is not in 100%, but 98%. Restarts can be blamed on "android.process.media" - problem just coinciding in time, but loony battery metering and phone not shutting down after battery drop to 0% seem to be kernel issues. Or the phone restarts suddenly, and after reboot battery level is 70% higher...
Similarly overheating and crashing while playing (Real Racing 2). I had no problems before. (I went to tweaks app and decreased GPU frequencies from the default. That helped.) Once again, I did no other changes in tweaks.
I'm not saying don't use Siyah - just backup your stock kernel (if you know how, because I don't, but that's another topic). And I would recommend not leaving the phone running on battery unattended until you are sure everything is as expected. I have to rescue my S2 anyway, and will clear the "warning" if I'll have same situation under PhilZ kernel and new battery.