I'm personally in the "say nothing" camp, but I know plenty of people who aren't, including several with histories of complex trauma.
I always feel like, in the moment, it's partly lack of self acceptance (which makes a lot of sense if you have a trauma history), and partly also a subconscious trust-testing exercise.
When someone dumps a whole lot of their trauma history in my lap unexpectedly, I can't help feel like they're expecting a negative reaction. Like I'm going to tell them they're crazy, or unlikeable, or that's gross, or plain old "you're too much". The underlying assumption being: people are untrustworthy, so I'm gonna make sure nice and early that I'm right about them...
If that's right (I think that may be from schema therapy), it's a coping strategy. A maladaptive one (because you freak people out, and sabotage your ability to form healthy relationships with healthy boundaries), but a coping strategy none the less. So, not something to be ashamed of, but to recognise and develop skills to work around, you know?
Could be totally wrong. Some people just like talking about themselves. But with trauma, it's less likely to be that. Because people don't typically like talking about their trauma.
In an entirely different realm, sometimes it's just cathartic. Getting it out of you, you know? It's still gonna sabotage your relationships, but also still not something to get stuck on. More like, "Okay, I do this, I don't want to do it, so let's work on boundaries and communication skills".