It scared me a couple of times when trying to find the optimum UEFI settings but nothing I could not eventually resolve.

I think I have flashed the bios at least 10 times. 

Have not tried skyoc yet, I'm using iGPU and frankly it runs every game I throw at it with decent framerate at 1920x1200 with low or medium quality in game settings

Bios 2.80 allowed me to set a BLCK of 103Mhz while the latest 3.10 is not stable past 102Mhz but the difference is minimal in games. 

It is a lot more important to try and keep the HD530 at its maximum speed without throttling.

Dang, ya know what? Right after I edited my post above, nolapic, I had a brain storm. Maybe Go into the BIOS, disable HPET, and retry.

Good thinking 99.  But no cigar unfortunately.

Thanks for your efforts nonetheless :)

I've also tried the automagical Sky OC "press X to overclock" feature (which seems to lock the OC to a speed that while higher than base is still less than the CPU's turbo boost speed? :/) and it doesn't behave any differently from a manual OC.  I suspected that this would be the case but I tried it anyway.

Vcore adjustments seem to be more reliable in the sense that they actually stick but since the problem was intermittent before only further testing will confirm an improvement.

I haven't tried OC'ing beyond 4GHz on the new BIOS yet to see if there are any stability improvements but it's kind of a moot point anyway if I can't boot into Ubuntu (I do most of my gaming in Windows, but most of my really CPU intensive tasks in Ubuntu).  There's probably just the right kernel argument (or combination thereof) out there somewhere to serve as a workaround but I haven't found it yet.

At this point I'd like to encourage anyone who stumbles across this thread via a google search to sign up and report on their own experiences with BCLK OC'ing on this same motherboard as it would be profoundly useful to know whether or not I am the only one having these problems.  Please :)

I will of course wait to hear back from Asrock support before I decide what to do moving forward.

Thanks for the input so far everyone :)

The Linux (Ubuntu 15.10) boot issue doesn't appear to be stability-related since even the tiniest of overclocks (110MHz BLCK for example) stops it from booting (hangs with a flashing cursor after grub with no useful information logged).  This suggests that the issue might be related to bad handling of a scenario involving disabled CPU features or something like that.

Knowing that Skylake support was basically sort of hacked together for 15.10 I decided to update it to kernel 4.3, but the boot failure persists in an identical fashion. Even a daily build of 16.04 behaved in precisely the same way.

Again all it takes to make it all work again is to set the BCLK back to 100MHz.  This is getting to be a bit of a deal-breaker.  As it stands it would have made more sense to get a cheaper board with a low-end chipset to pair with something like an i5-6600.  Roughly the same outlay, roughly the same performance (as my 4GHz i5-6400 OC) but without all the drama.

Still hoping the situation can be salvaged with a BIOS update of course.

Done :)

Enabling the Max Battery option is itself interfering with the overclock by virtue of locking the multiplier to a low value.  Yes, that is appropriate behaviour, but only up until the point where the OS has booted. But for me it remains locked to x8 even after Windows has booted.  This means that a 4GHz OC becomes a 1.2GHz underclock (while the baseclock remains at 150MHz).

Setting it back to "Max Non-Turbo Performance" gets me back to 4GHz as a result of the multiplier being locked at x27, which is where it should be.

"Max battery mode will set CPU ratio as x8 until OS handoff"

Given the description, and the fact that it is the recommended setting for BCLK overclocking, one would have to conclude that this is a bug, right?

Why bother with the setting then?  My thinking is that it might solve some intermittent boot issues and help facilitate higher overclocks (by putting less initial strain on the system), so it seems like a useful feature.  As such I'd like to see it working properly :)

Sure.

I haven't checked the log yet.  But the system has no trouble coming out of sleep when I'm not overclocking.

The BIOS on this board is sparsely populated with settings compared to some other Asrock Z170 boards.  I've disabled things like Speedstep and Turbo Boost obviously.  I keep the RAM within spec.  I keep the vcore at or close to 1.325v. About the only "special" BCLK OC-related option is the one that doesn't work properly: "Boot performance mode".

Yes, it boots fine when everything is running at stock speeds.  It also wont boot with the "Max Battery" option, which as described above seems to result in a permanent (for the boot cycle at hand anyway) underclock.