Interestingly, Zoro and Sanji share a similar flaw, but different versions of it!
You see, Sanji thinks himself worthless, unless he can be of use to the people he cares about, that's something developed because of all his trauma growing up. So if a situation happens where Sanji could take his own life just to try and be useful for someone else he cares about, he would do it.
Sanji's insecurity created the events of Whole Cake, where he didn't want to be a burden for the crew, and because he thinks so little of himself, he decided to go, just to not be a burden and keep the crew safe, being useful to them.
That's why in Wano, after his character developed and listening to Luffy talk about him in Whole Cake, Sanji decided he has value just as himself, without having to sacrifice who he is in order to be useful. Sanji doesn't need to become the science perfect warrior that would be super useful for the crew, because he knows as Sanji he has value and that Luffy and his crew likes him for just being Sanji, not an incredible warrior machine.
Zoro is the other side of the coin. Zoro values himself very highly, yet he keeps finding situations where he isn't enough, moments such as his fights with Kuina, the fight with Mihawk, the first fight with Lucci and Kaku, both fights with Kuma.
This insecurity, of failing despite of thinking yourself so great, is what ends up making Zoro train like crazy, because he wants to be strong enough when the time comes.
In many of these moments, when Zoro sees himself not raise up to the challenge, he almost "gives up", he surrenders his life in a way, because in his head if he isn't good enough, then he might as well die. This happens a lot.
Two of his most famous fights, Mr. 1 and King, both have the same theme and conclusion in a way, where Zoro starts the fight and right away realizes he might not be strong enough right now to defeat his enemy, but he has to overcome this hurdle to win, and he has to risk his life to reach that, because if he fails, whatever, he might as well die. But if he succeeds, then he was strong enough.
So both characters end up sharing a flaw of not valuing their own lives as they are. They only think they are worthy of living if they do what they want to do. For Sanji, that is being useful to the ones he cares about and For Zoro, that is being strong enough.
However, because of Whole Cake and Wano, we now understand why Sanji had this need of having to be useful and how he overcame this flaw in Wano after Luffy's words to him in Whole Cake.
But we still have no idea why Zoro feels this need to be strong all the time and if he will ever overcome this idea that his life has no value, if he doesn't reach this level of strength he seeks. Maybe he was just born with this endless desire to be strong. Or Maybe something happened before the start of his flashback, some kind of moment where Zoro was not strong enough and suffered for it. Because remember, Zoro already wanted to be the strongest before he even met or fought Kuina. So he either was just born with this will or something caused it.
In any way, that's a flaw.
Zoro thinks his life is not worth it, if he is not strong enough to become the World's Strongest Swordsman.