In every iteration, as far as I can recall, Brave has been overwhelmingly positive. And obviously same goes for Ambitious, etc. There's an argument to be made as to whether all traits should have interesting pros and cons such that you are choosing between different
types of characters, or whether it's more about having a lot of traits that might be great to terrible but you can't control which you pick up so you have to roll with the punches. CK2/3 does a bit of both, of course, but it tends to be very easy to control your child's education and genetic traits - even without spending a lot of lifestyle points on it. So if we're going to give players so much easy control, then I would suggest that all/most traits should come multiple-edged.
I think Brave does increase chance of combat deaths. At least it did in CK2 and this was entirely unlabled. The thing is you just... don't have your king fight in combat, because even if craven has 1/10th the chance of dying as Brave, why would you risk it? And every commander becoming Craven after fighting a few battles in CK2 was just annoying bullshit honestly.
Well, that's why I think it shouldn't be such a given that your King living until 90 every time is a great thing, and Brave is always better than Craven, and so forth. IMO it's really jarring and boring to have your realm's history reading "King X the Brave, ruled from age 16 to 80, kept his 25 Martial all the way through, succeeded by King Y the Ambitious, ruled from 20 to 70..." which is what happens easily even if you aren't trying to munchkin.
If, for example, a Brave king acquired massive stress from not being out on the field, and you had to choose to put him at some personal risk of injury / craven / etc, and stress actually mattered more, then we'd also have a game where a lot of your characters
do pick up multiple stress traits if they've been around for decades and it really does crimp your style. As opposed to now, when I get a Mental Break and I'm just like whatever, half the time it just gives me more bonuses.
All of these mechanics already exist in CK3 and did in CK2 - they just stay at a very mild level so you can lean into that and larp your flawed King, but the base mechanics essentially sit there and say "do whatever you want Oblivion style and be great at everything and live forever!"