From my past experience of FlexRaid user, I think FlexRaid is not reliable enough. Reading the forums, there are still too many issues when trying to restore a failed drive, whatever the reasons are (update of the parity drive not done or failed, disk errors, etc...). I lost 700+GB of data, all recovered files were corrupted and it took ages to recover these corrupted files. No error during the last update during the parity, etc... I was using FlexRaid with HD Sentinel.
This is my own experience. It might work perfectly well for other users - even if I am wondering how many people already tried to restore a 6TB drive to check if everything is ok.
So I decided to sacrifice hard drive space in order to be sure that my files are protected and that I can recover them without any issue and not waiting for x days: I moved to DrivePool and Scanner. I am pretty satisfied until now even if I have some small issues with Scanner (some of my drives are not recognized). But I like the integration of these 2 tools (files placement rule based on Scanner).