Yonko level Yoki was Father's true biggest threat.
It was indeed a problem with adaptation with Yoki's storyline being cut down, though it was an excusable cut when when he still got a mention in the newspaper and from secondary sources in the first and second episodes to establish who he was.
In fairness he even received a flashback further on, O believe after Limblee nearly killed Ed by the mines. But it was extremely rushed, and it felt like his character was shoehorned in for no valid reason. Even a slightly longer segment on him would have done the trick.