it's only viable on knlontas mire in the various weird chokes
People still play that map?
When going ling/baneling/muta I would agree with not using vipers against non-siege tank bio. They have the potential to pay off and work out, but in most situations having a few more infestors and ultras is better. Keep your mutas alive as long as possible to kill medivacs. When the mutas die, make corruptors and continue killing medivacs in large battles. Once the terran's medivac count is reduced, morph brood lords.
Ultras are a stronger mid/late game unit than broodlords, but eventually terran manages to make enough marauders and mines to deal with them. Fortunately broodlords hard counter them, so they are a great tech switch late game.
It all comes down to where you prioritize gas spending. Personally, ling, baneling, muta, infestor, ultra, corruptor, and broodlords tends to eat my gas throughout the game.
If your going roach hydra, vipers are required. When zerg has a supply advantage they can take fights against bio with roach hydra, but when terran catches up in supply zerg must have vipers to compete.