It's not that Valkyr isn't viable. The new changes address her primary problem, her basically nonexistent survivability outside Hysteria.
Her problem is that she doesn't DO anything.
The game has, admittedly, tons of balance problems, especially with damage scaling, but the issue is that Valkyr, Oberon, Saryn etc have little to bring to the table.
Okay. You can make yourself invulnerable, heal from mele strikes and gain a mele damage bonus?
Yeaaaah. My Penta-using Trin would like a word with you.
I've said this in the Warframe forums and I'll say it here.
Due to enemies always knowing where you are (bar on a stealth frame, and good luck playing 4 of those in a long defense or survival), the level design being constrictive and enemies being able to take out massive chunks of health/shields when they get over lvl40, the entire metagame revolves almost solely around damage negation: CC abilities, invulnerability and damage canceling. It's why everyone and their dog in Recruitment wants Trins, Vaubans and/or Frosts. It's actually why I've got faith that Zephyr will be viable from the get-go due to her 3 and 4 which fit into the current metagame nicely. I was actually bouncing a Warframe idea off The Darkling Wolf a while ago and something akin to her 3 came up.
Valkyr herself isn't "bad". There's plenty of spots where I would see her being useful. In other games.
Again, I'll say the same thing here as I've said there. Fixing the systemic failings of the metagame would require a massive reworking of what Warframe actually is, and perhaps it isn't worth it. Bringing frames into alignment with the metagame, however, can be done. Make Decoy and Molt not useless and give other frames more inventive or at least convoluted cheap CC. Give more frames party-wide damage amps, because zapping one perma CC'd Bombard for half a minute until it decides to relent and fall over dead is the antitheses of fun. Fix damage scaling on abilities etc.