The only problem with Black Flag is all the Abstergo shit which is dreadful and the story which is at best pointless. The gameplay itself is great though. I'd definitely recommend playing it.
As for Origins, my main problem (been a while since I played it) is that the main story structure was very outdated: "You have to kill three targets, you reach the checkpoint, now you have to kill another three, another checkpoint now another three". It's how the original AC was constructed 10 years prior.