The RAID 5 concern is not as much the incidence of a 2nd disk (full blown) failure, but two unrelated problems that negatively affect a rebuild should one drive fully fail. One is a read error, which is the actual loss of a sector of data and usually reported as a read error with no other result returned to the kernel. The other is a corrupt parity chunk which goes undetected.
A sector read error during rebuild will cause rebuild to halt. You can try to restart the rebuild and maybe it won't happen again. If it's a persistent read failure, the array is toast, and will have to be created from scratch and data restored from backups.
A corrupt sector that goes undetected by drive ECC will result in corrupt reconstructed data. And that will propogate into your backups, etc.
Anyway, for these and other reasons, I consider RAID 5 to be more trouble than it's worth, for those who care about their time and data, when used with consumer hardware.
Professional photographers in particular, and quite a few amateurs, produce an amount of data that a business of 100 or 1000+ produce. Of course those businesses have dedicated IT people to handle their storage requirements. Photographers don't. As a consequence there's a gap in good storage solutions, both reasonable cost and reliable.
As for disk testing, again nearline and enterprise disks are usable right out of the box, as they're tested before they're distributed. For consumer disks I use a combination of several SMART extended foreground tests (read only), checking SMART attributes for concerning changes, interlaced with either ATA Secure Erase (write and read) or writing zeros. ATA Secure Erase is faster since it's done by the drive firmware.
I'm not aware of any GUI apps for OS X that do these things. The defacto standard for monitoring and testing SMART drives, smartmontools, are CLI only programs. Binaries exist for Linux and Windows, but not for OS X for some reason. Instead smartmontools is available for building from source via Macports. I think most people would find it easier to boot from a Linux LiveCD that has smartmontools and hdparm (used for ATA Secure Erase) already on it, assuming they aren't already scared away.
Anyway, I tend to be more of a fan of a NAS based storage, Linux or FreeBSD based, which will have these tools integrated and can be scheduled, as can regular array scrubbings. NAS 4 Free combined with ECC memory, and nearline drives, is a pretty robust and scalable solution. You do give up some performance over Thunderbolt or USB 3 solutions for certain operations but I'll argue for other ways to mitigate this rather than advocate large locally attached storage.