Easiest class rankings?

[Tsumdere]

All of those outcomes make perfect sense.
If there is one thing where people lose most of their dmg, even in the current tier, its still good old uptime. RDM with its castbars is still able to lose more uptime than most jobs, thats why in the current tier it can be a bit harder to play optimally.
DRG has been consistently the second to third highest variance behind BLM, which may come as shocker to some people due to how simplistic it is, yet it has been known among the more hard core crowd to be difficult to play optimally just due to its rigidity and strictness.

Yeah, you kinda just agreed with me that variance is not a good metric and doesn't support the community perception.

DRG is simple and static, but punishing in fights and if you mess up rotation. This applies 100% to MCH but the variance is extremely low. This change in variance is because most of MCH's big hits are auto-crits. If Wildfire could crit and Reassemble didn't exist, you would see MCH's variance widen drastically. But this would not change anything about the rotation or difficulty of the class.
This can be reversed where you could add more auto-crits to DRG and BLM and their variance would go down.

I absolutely think RDM is one of the hardest classes because of uptime. But it also has increased variance from rezzing or (god forbid) emergency heal for the clear. RDMs are very affected by bad runs where they drag people across the finish line. This is not indicative of their rotational/optimization difficulty.

And naturally the difficulty of optimizing jobs would shift due to encounter design.

Yes. Difficulty is subjective and different per encounter. That's why variance is a flawed metric and cannot be used as a definitive way to rank classes.

You'll notice how much DRG spikes in DSR. It's because it has two target phases. This is not changing the difficulty of the class, but rather adding another target to crit on and DRG are good at cleaving. You can see this yourself by seeing when the variance spikes in the fight - Eyes (as DRG is only class that can cleave both) and Double Dragons.
SMN spikes because SMN is extremely good at non-gauge burst and the bosses jump a lot. Crits mean a lot in those burst windows.

You'll be very hard-pressed to find people who believe optimizing SMN in DSR is harder than RPR/BRD/MCH/RDM (all notoriously nasty to opti in that fight).

[ChaozK]

DRG is simple and static, but punishing in fights and if you mess up rotation. This applies 100% to MCH but the variance is extremely low.

Funny you mention MCH and DRG as these happen to be my two main jobs. I can 100% tell you that there are worlds between MCH and DRG in terms of how bad play affects these two jobs dps. Most of the stuff that makes MCH very intricate has comparatively minor impact on their overall dps. This is what makes it more forgiving.