Like i said earlier, preserving buffs is really not a problem for the viper, and actually i don't think that the gauge is a problem either, i've killed FRU multiple times and always felt that my buff management and resource was pretty smooth. No, I think that, like you said at the end, the problem is just that those jobs usually have time based skills that are too weak : viper's vicewinder is clearly too weak, machinist drill/air anchor/chainsaw are too weak but should not be buffed too much because drill is the biggest part of machinist dps, samourai looks fine but should actually be a bit higher too, and reaper same as samourai.

I also think that all those jobs should have a higher burst, except maybe samourai who should maybe buffed on his filler or odd minutes instead, but viper, machinist and reaper burst phases are clearly too weak looking at how overwhelmingly strong every raidbuffs are. Again the only melee job that has a weak raidbuff, reaper with a 3% raidbuff, is clearly the weakest with the two selfish jobs, samourai and viper. There is clearly a big problem in term of burst damages between selfish and buffers, and this will never be balanced if selfish don't get a burst that are at least as strong as jobs that have a raidbuff like ninja and picto.

And i'll add one more thing for the viper : actually, i'm pretty sure that the reason why he's weak is because of uncoiled fury. Why? Because since the beginning, SE has been balancing this job with the idea : "ok, viper should never lose uptime, so how should we balance it so it isn't too strong in comparison to other melees?". This is actually a big game design issue, from the beginning i think that the viper should have never been allowed to get 3 stacks of uncoiled fury at the same time, because it is just unfair for other melees, no melee should have a tool that allow them to get so much uptime in comparison to others, and since SE knows that it's unfair from the start, they have given the viper weak damage overall to compensate. We've already seen what happened at the end of patch 7.05 : viper was the weakest melee(with the reaper), and now we see what happens on FRU, viper is just the weakest overall, even when we consider P4 where he is actually supposed to be broken.

I think viper should be allowed to get only 2 uncoild fury at the same time, and i think maybe serpent's ire should not even give a stack, not sure, i always felt like this skill should not give one. But on the other hand, viper should get a big buffs on his reawaken and vicewinder combos.