Well there has always been the chance that upgrading the BIOS can and sometimes does Brick your notebook. Following these forums I would say so far the G752 have been the least likely to brick. The G75 VW were no issue until BIOS Version 221 came out then we were seeing almost daily people with brick Notebooks from BIOS upgrades. Some were rescued with the Ctrl + Home many were not and had to RMA to recover. While there have been a few through the G75VX, G750, G751, it hasn't been a lot of people that actually bricked there notebooks.
I think your probably correct that using the older winflash may have caused your problem, Or it could have just been bad luck. Also one of my always do is to unzip the download directly to the media I intend to use up update with. Copy and Paste that file can cause slight changes that may or may not brick your notebook. Notebooks are just not like desktops that we all have upgrade BIOS/UEFI with out problems for years and years. Seems since UEFI it has become a little more flaky and hazardous not to be taken lightly. I personally never have updated my G75VW beyond 210 no one could tell me of any improvement in 221, 222, and there were lots of Bricked notebooks.
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro