A few comments:
S/A/B/C/D/F rankings aren't really appropriate for ranking Draft archetypes.
First of all, it's not a fighting game where S-Tier are overpowered dominant forces in the meta, C/D require a character specialist/master to actually win with, and F is completely unplayable at any serious level of competition. It's a booster draft. When three colors are being focused by the other players at the table, it inherently makes the remaining colors stronger for the few that are getting the best cards in higher concentrations. This fact alone basically eliminates the possibility of an F-Tier existing.
Secondly, the parity between archetypes in BFZ is a lot closer than these rankings imply. I would personally agree with you that U/R is probably the best and U/G and W/G are probably on bottom, the gap between them certainly isn't S-Tier to F-Tier. I don't think U/R Devoid is as dominant as you say and I don't think U/G and W/G are unplayable jank like the rankings imply. WG won the Top 8 at GP Madison (although I didn't think his deck was very good.) To me, your order is mostly agreeable. I agree with your top and bottom, but would rearrange some of the ones in the middle. Truthfully, all ten archetypes (defined by color pairings) would actually all fall within A/B/C Tier.
WB Lifegain should be ranked higher. It can basically be drafted two ways:
All-out lifegain synergy with multiples of Kalastria Healer and allies to trigger them, on top of other cards with recursive lifegain (eg. Stone Haven Medic) and cards that trigger off of lifegain (eg. Bloodbond Vampire).
A deck full of WB "good stuff" with incidental lifegain synergies that are tacked onto otherwise decent cards. WB has one of the best removal suites (second to WR), and its roster of evasive creatures is nearly on par with WU, which is saying a lot. Add in synergies from Kalastria Nightwatch, Bloodbond Vampire, Serene Steward, and you got a solid deck that isn't overly dependent on the lifegain synergy.
I think it's the most misunderstood archetype. People have this idea that WB is completely dependent on getting all the lifegain cards. I think a better approach is to simply draft strong W/B cards early and transition into the all-out lifegain if the cards cooperate. That way, you'll always have the "good stuff" deck to fall back on. There are tons of overlapping cards anyway.
You can't force UR Devoid. Or any archetype really, for that matter. When the format is centered around synergy, if multiple people force UR Devoid, you'll end up with watered down versions of what the deck should be. You can lean towards it if you'd like (I lean towards Red), but in a synergy-heavy draft format like BFZ, you're better off identifying open colors and reaping the benefits throughout the second and third packs.
Green is the weakest but it draws its strength from non-Green cards. If you asked me what the weakest color is, I would say it's Green. However...
Green is most able to splash into other colors due to cards like Fertile Thicket, Blighted Woodland, Lifespring Druid and Natural Connection. It is rewarded with cards like Skyrider Elf, Tajuru Stalwart, and Infuse with the Elements. If you're in GW and open Munda in Pack 3, you can play it. If you open Drowner of Hope or Kiora, you can play those too. This is still possible with non-Green pairings, but more possible in Green.
Green is also most able to cast Eldrazi due to ramp (Lifespring Druid, Natural Connection, Scion producers). So, while the Green cards are the weakest, expensive Eldrazi cards are more Green than they are White/Blue/Black/Red.
Also, I think people are not playing Fertile Thicket properly. You have the option of bottoming all five cards. If I don't want to draw another land unless it's a Plains, I can purposely "fail" to find a Basic land toss all those Forests and Mountains to the bottom. People are either triggering its ETB to find their missing land and begrudgingly putting a different basic on top. Or, they're declining its ETB trigger, which you should never do unless you have some Scry action going on.