Raven: “You said Tai told you all about my semblance. ... Well I doubt he ever told you what Oz did to my brother and me.” (V5Ch 6)
Ozpin: “Everyone has a choice. The Branwen's chose to accept their powers and the responsibilities that came with them. And later, one of them chose to abandon her duties in favour of her own self interest. (V5Ch7)
So Raven and Qrow can turn into birds. Raven heavily implies that's bad somehow, but we've seen Raven and Qrow transform multiple times and both seem to find the ability helpful and suffer no ill side affects. So why frame it as a bad thing? I wanted to look at this without dismissing it as bad writing, especially since most critiques I've seen claim the writers were trying (and failing) to portray Ozpin as evil which is flatly ridiculous. There is no point in RWBY where Ozpin has been framed as evil. Flawed yes. Evil no.
Ok, so why? Well, Raven could just be lying. She doesn't like Ozpin and is being overtly manipulative. She turns into a bird in a clear display of magic in order to push Yang towards her side of the ultimatum. It doesn't work, but I also think there might be a little more to it than that.
When Yang accuses Ozpin of doing something wrong based on Raven's words, Ozpin is able to laugh it off while Qrow assures everyone that this was a choice he and Raven made. However, given that we learn how much Ozpin was hiding from even Qrow in the next volume, neither Raven nor Qrow were able to make an informed choice. It's possible Raven doesn't resent the power, but the implications. The way it ties her to Ozpin, the way it could potentially put a target on her back, that she made a choice before she knew what was really going on.
This is especially interesting because Raven is also a Maiden. Both the ability to turn into birds and the Maiden powers are framed as “gifts” from Ozpin/a previous incarnation. Interestingly, only the Fairy Tales of Remnant book version of the the origins of the Maiden's makes it clear that accepting the magic was a choice. Even if the first Maidens were able to choose, there are still countless women and girls who weren't. I suspect Ozpin did not intend for the Maiden powers to pass on from host to host, but he is ultimately responsible.
There is also the matter of the Vaults. At some point, and it's not clear when, the Maiden's became keys to the Vaults. Now Salem has a direct incentive to go after the Maidens. Most women who become Maiden's almost certainly know nothing about this. Even in special cases like Pyrrha, where she did get a choice, there's so much important information Ozpin didn't tell her, and it's unlikely he was planning to, since, once again, he was hiding a lot even from Qrow. Ozpin puts such and emphasis on choice, but it's been made clear that the choices he presents are not always honest.
Yang infers that Raven killed the previous Spring Maiden to steal the powers, but Raven says it was mercy. Since we have an onscreen example of Maiden essentially being mercy killed via Penny, perhaps Raven wasn't lying. If this wasn't something she actually wanted, if the Previous Spring Maiden was someone Raven cared about, seeing the tole Ozpin's actions and secrets have had would be enough to make Raven more than a little resentful.
Regardless if that's true or not, I think Raven sees some parallels between the 'gift' Ozpin granted to Raven and Qrow, and the 'gift' of the Maiden powers. There's definitely some strings attached to both.
