Despite all of his obvious flaws, Andras is perhaps a better leader than Jezera. Sure, he is in many ways primitive and brutish, also has anger issues. But Jezera is even more incompetent: despite being good at scheming (and often succeeding at it), she is completely childish, irresponsible and seems to be less focused on her duties than Andras. The red demon spends most of his time in barracks, training and commanding orcs, while also supervises forge work and driders - overall he seems to be very focused on his duties. Jezera on the other hand tends to spend most of her time playing with maids or devising petty schemes. She doesn't seem to be taking her role seriously at all. While Andras often cooperates with Rowan on his missions (we can see it during orc tribe events and village occupation events), Jezera spends most of her time in her chambers. If Rowan contacts her during his missions, she is interested only when it looks enjoyable to her (e.g. how she helped Rowan with corrupting the traveling nun or how she came to the witch convent). In other times, she simply doesn't care. During the event with Darla the Huntress, when Rowan contacts Jezera to consult with her a possibility of recruiting a large group of mercenaries, she tells him to stop bothering her because she is currently having some fun. I don't remember Andras being ever so negligent, despite also liking to have fun.
Also, Jezera's schemes, even if they succeed, are often very one-dimensional. Sleeping with an enemy general is a nice idea, but can succeed only against a man of lesser character (I sincerely doubt that someone like Werden could have fallen for a trick like that). In the prologue, Jezera lures Rowan away from Arthdale to capture Alexia, but when Rowan returns she is caught by surprise and is nearly killed/wounded by him. Even Andras calls her out on that after he comes to rescue her. We could also argue that each of the twins has an area of responsibility. Andras is in charge of the military and munitions, Jezera is in charge of spies and foreign relations. But whereas Andras does his job very well, Jezera is only partially successful. She succeeds in gaining Ameraine as an ally in Rastedel but miserably fails during her negotiations with the dark elves. Also, despite being a spymaster, she completely fails to stop plotting of important castle staff members. Both Cliohna and Shaya manage to take Rowan (who is supposedly monitored by Jezera) under their control in their respective paths (the only one who can stop them is Rowan himself, not Jezera) and Cla-Min is running wild with corruption, counterfeit money etc.
