TBH I'm a filthy casual so I literally only ever use Ember's 4 (and occasionally her 1 for breaking those hidden crates in the Orokin ceiling dome things). Which strikes me as... probably not the intended way to play? But there's already so much to pay attention to in Warframe and I'm bad at the mathy optimization stuff so when they started going on about managing this meter I just gave up. I still use her for everything, but that's because she's my most developed/forma'd frame either way, and her 4 makes a good okay "oh crap" button, and I'm fundamentally a creature of habit.
I'm still sad they changed her passive. Not for gameplay reasons, because quite frankly I don't understand those very well, but because "gains energy when on fire" is METAL AS HELL. I mean, imagine you're some Corpus grunt trapped in a dead end by debris from an explosion, hoping the Warframe that killed all your buddies isn't going to get you next. You think you're relatively safe because even though you're surrounded by flames and you can't get out, that means the Warframe can't get in, right? And then you see it look straight at you and deliberately step into the fire. And not only does it not seem at all hurt, it's actively getting stronger. That's way scarier and way cooler story-wise than... I can't even remember what it does now, a conditional power strength buff or something, IDK. It definitely doesn't change my behavior like the original did. I basically ignore it because it doesn't do anything a mod can't do.
(Oberon had the same thing happen. His passive used to be that wild animals within range would be neutral/allied to him, which granted was back when that just meant the feral kubrows and kavats in the Earth/Orokin levels, but can you imagine how cool that could be in an open world environment? It was unique, it was on-theme, it fundamentally changed how I interacted with the game... and now it's just a companion buff, which stacks with the other companion buffs. Bleh.)
Agreed about the directional LOS thing, though. It would make sense for, like, Chroma, as a dragon breathing fire, but Ember? Why would fire only go in one direction? However, I think it could still work with a 360 LOS limitation. Think of the center of an explosion, or a light: if something's in the way, it casts a shadow on whatever's behind it. If there's no direct path from Ember to the target, it makes sense that she can't magically set it on fire, but it's not like she's got a cone on her head like a dog after surgery.
Besides which LOS barely works as intended. Whether or not enemies behind a wall get hit seems to be basically random, leaning to "when in doubt, yes". I mean, I'm in favor of that, because it makes it way easier, but it's not really in the spirit of the thing, is it.