Fugger did a good review of the i7-5775C over on XS:
http://www.xtremesystems.org/forums/showthread.php?291735-Broadwell-5775C-on-Phase
Not everyone cares about OC on phase, but if you pick through the thread, you can find lots of data on how well the thing performed with a mere AiO cooler. That was also some time ago . . . support for the chip is certainly better now, and Intel might have some better retail samples in the wild now that they've had a lot of time to work on getting yields of 14nm up a bit.
Anyway, that should tell you what you need to know about Broadwell-C overclocking.
The issue is that they didn't do it out of the gate.
Most people regard broadwell-c to be pointless anyway, and given its poor availability, it is mostly a non-factor in the purchasing decisions of buyers.
http://techreport.com/review/28751/intel-core-i7-6700k-skylake-processor-reviewed
As you can see from techreport's findings, the 5775C still hangs close to the 6700k in many workloads. The uarch improvements in the 6700k and the higher clockspeeds should have let the 6700k walk all over the 5775c, but it doesn't. A chip that has a 3.3 Ghz base clock has no business hanging that close to the 6700k. It's weird.
The only really "fast" iGPUs Intel has right now are its eDRAM-enhanced parts. So once DX12 shows up and starts throwing compute functions at the iGPU, the parts that should take home the prize are the fastest ones . . . and for Intel, right now, that's the 5775c. Though I must admit, I was a little confused by some of the Luxmark performance on the 5775c.
Do not expect DX12 or Vulkan to have that problem.