but, I haven't flown a FlyDubai roster. . . . . . .
I have.
I have had rosters very similar to the poor FO who died at Rostov, and on one flight was so tired I managed to forget the phonetic alphabet and even struggled to read our callsign back to ATC. Until you have experienced that level of fatigue, you cannot appreciate just how much it drains your capacity - and how reliant you are on the person sat next to you.
Many of us felt it was only a matter of time. Just put two equally fatigued pilots together in the middle of the night, throw in some bad weather and/or non normals and you're rolling the dice. It could have been me on that flight, or any of my mates. My only surprise was that it happened at Rostov - I can think of at least half a dozen places on the FZ network more likely to witness a smoking hole.
There but for the grace of God.