About Blake :
I never seen her as a coward. Sure she run away but not because she afraid. She run form her parent because she believe in the white fang cause, she run form the white fang because she disagree with their method and she run away from team RWBY because she think it was the best way to protect them.
Those aren't the action of a coward, on the contrary it's require some courage to leave the comfort of your love one for a greater cause, who'd you call Oscar a coward for abandoning his mother alone in her farm ?
I don't think Yang think she a coward either, she just angry at her because she can't process someone leaving her (for obvious good reason). More than anyone Yang need to know why Blake left and once Weiss give her a reason her anger turn to fear that she might not come back, fear that she fail to help her when needed.
Now that team RWBY is back together the only thing Yang need to forgive her is the assurance that it won't happen again which she'll have because Blake know she made a mistake (thanks to Sun) like she know she made a mistake not listening to her parent back when the white fang change politics.
About Raven :
If she run just because she was afraid of Salem then she a coward. But I don't think it's that simple. Sure she most likely afraid of Salem (like almost everybody who know her) but she also lost hope, she didn't believe in Ozpin cause.
We don't know what Ozpin ask Team STRQ to do, but it's look like they was on the offensive (unlike know when they just defend their city, so far).
Raven didn't believe Ozpin and co can win against Salem so she leave the front-line to take a defensive stand even if that means abandoning her daughter. Forcing Tai to leave the fight too because he had raise their child alone.
If you were a soldier who desert because you believe your general have the wrong tactic and going to send you to your death does that make you a coward ?
Only if you didn't do something, if you didn't come with a better plan, a better tactic, that prove you were right and the general was wrong.
The fact is Ozpin optimistic tactic have fail, this trust in general ironwood and Lionheart lead to hack robot and treason, his teaching method for his student lead to Pyrrha and Amber death and before that the flee of the former spring maiden and Summer death. His symbol of union in the Vytal festival lead to the fall of Beacon.
While Raven ruthless tactic have success. Her way of leading had make her tribe stronger than ever, people capable of surviving in the wild even when it's full of grimm. Her heartless choice to kill the weak allow her to find the spring maiden before Salem and Ozpin. Her despicable method of lies and deception make possible to lure Cinder into a trap, kill her and secure the relic.
I know I'm being relentless on Ozpin and understanding on Raven but I try to make a point. War is ruthless, unfair and ugly. Unfortunately in war the one who survive are the one like Raven not the one like Summer.
That's the ugliness of the world, the bad way works better than the good way.
However I still believe in Ozpin way. The heroes way. Heroes are stupid, unrealistic and candid but that make them able to do thing everybody thing are impossible to do. The only thing the good one can do better than the bad one is defeated the evil one.