Since starting to use the fvdw firmware on my 5big vs2 about 2 years ago I've been unfortunate enough to experience 2 drive failures. One was on v16.1 and one on 17.0. I very much appreciate the ease that 17.0 added to performing the drive replacement, however I noticed both times that when logged into the web interface while a drive was actively failed, on both versions it stated "Current System Status: No Alert" and I didn't get an alert email about the drive failure despite configuring that feature.
I would think that a failed drive is the HIGHEST possible alert condition there is other than maybe the NAS processor overheating enough to destroy itself and so was confused by the system status and lack of email. Luckily my NAS is somewhere I see it on nearly a daily basis so it didn't go unnoticed for long in either case because the front round LED button blinks red, but if someone had theirs in a place that they couldn't easily see or disabled the front LED button they could have a failed drive go unnoticed for extended periods of time. Not sure how difficult implementing this would be, but just wanted to throw the suggestion out there.
If you'd need help testing and could have a way to safely simulate this (without risking my actual data by randomly pulling a drive or something) I'd be more than willing to help test with my 5big vs2.