The Rishonim debate the parameters of their opinions.
Rashi indicates that Rav only allows one to back out of a verbal commitment when conditions changed, as in the case of Rav Kahana.
Many Rishonim, including the Rif, Ramban, Tosfos (49a Modeh) and Rashba hold that Rav and Rabbi Yochanan hold their positions regardless of whether conditions changed. In all cases, Rav says it is considered trustworthy, while Rabbi Yochanan says it is considered untrustworthy.
The Baal Hamaor and the Rosh say that both Rav and Rabbi Yochanan allow one to back out of a verbal commitment if conditions changed.
According to the Baal Hamaor, even Rabbi Yochanan would agree to the ruling Rav gave Rav Kahana, and the Gemora only used the story as a springboard for the more general debate.
In the course of the discussion, the Gemora quoted the statement that we learn that one must keep his “hin” (yes) just, by keeping his word. Abaye deflected this as a proof to Rabbi Yochanan by limiting this requirement to one meaning what he says at the time he says it. According to the Baal Hamaor, Abaye’s statement is also relevant to Rabbi Yochanan, since he allows one to violate his verbal commitment if conditions changed.
The Nimukei Yosef explains that in any case Abaye’s statement is relevant to Rabbi Yochanan, since Rabbi Yochanan agrees that one may violate a verbal commitment on which the recipient did not rely (e.g., a large gift). Therefore, the Rif quotes Abaye, although he rules like Rabbi Yochanan. Once someone is called untrustworthy, the community is allowed to employ social sanction, by calling him wicked, and announcing in public what he did.