Bokuho is also an offensive tool, because Helena can not guard while in it. She is wide open to an attack, and someone who is smart will go for it. Bokuho is a very fast stance to go into (like ~5 frames), and teishitsu can immediately be activated while in the stance. She has a 19 frame low that stuns (and has tracking), and allows follow ups to connect for a combo, she has a 12 frame mid, a mid kick launcher, two mid punch launchers, a high launcher , transition into backfacing, throws, etc. While she doesn't have mid tracking moves from the stance, 6p4pp is very hard to interrupt with a sidestep move. Overall Helena has tons of options from bokuho
Flamingo has no direct way to access it, except through a 4f+k cancel, and it takes more than double the frames to go into than going into bokuho, which means she has very few stuns in her normal stance that allow her to transition into flamingo and continue the stun for a combo. Even if she can transition, all options are slow escapable. However, stuns such as 6p4k, 1h+kk, and FLA 1kk, which start in her normal stance and end in flamingo allow her to transition into standing after the hit, and get a launcher, or if she is lucky, a critical burst.
Certain moves that leave her in flamingo give Sarah frame advantage, but her game while in the stance is limited. She has three tracking moves from the stance, but only one makes sense to use (3pk), because the others are too slow, (2h+k) or leave her at too much of a disadvantage on block (h+k). That leaves her with one viable tracking option from flamingo, and that move is a high. This means Christie has no problem sidestepping Sarah's flamingo mix up whatsoever.
She has one viable mid poke from the stance, and that is her 6k, which is 13 frames. Her two launchers from the stance are mid kick, she has no low that allows her to start a stun for a combo. People use FLA 1kk, but the second kick is counterable if you react to it. Her 2h+k is a low that is like 24 or 26 frames, and is very easy to see. It also flips the opponent in the air, and doesn't allow a follow up. She cannot execute a throw from the stance, or crush. Most people when they see Sarah go into flamingo do 1 of 2 things. They either sidestep, or they counter high. That completely erases 95% of what she can do from the stance, since she cannot throw them while in flamingo. Doing either of those things while playing against Helena is extremely dangerous since she can do everything while in bokuho except counter, guard, and low throw.
And if you think Sarah has tracking, think again. She has her sidestepping mid punch, her kk string, 8h+k, 3pk and 3h+k. All those are standalone moves. All of them are high or low, except for 8h+k. This means that Sarah cannot just whip out a tracking move like Helena can because a good portion of Helena's tracking moves start a string. Sarah can only rely on two things to win, and that is speed, and frame advantage. Sarah used to cause me so much grief until I started using her back in February, and now she is my secondary, and until Ultimate, she was my favorite character to use. Fighting another Sarah never really causes me trouble anymore, since you become aware of how limited she really is. She is also highly technical, like Helena and Rig, and takes a lot of skill to use. (It took me 3-4 months to learn her, meanwhile I learned Kasumi and Alpha-152 in a day). So if you lose to a newbie who is using Sarah, that is your own fault. If you lose to a good player using Sarah, say good game, because she is not easy to use.