Aside from a superficial association of offices and being allies, they are hardly a unit or a couple of paired characters who need to be analyzed together in order to grasp their significance.

By herself, Leane is a nobody. A decent and fairly competant woman pulled down by a friend she trusted blindly, in whom she resumes placing her blind trust as soon as they are back in Aes Sedai society. Whatever her individual and distinct traits might be, they are rather irrelevant until she starts exercising something like free will. As it is, she IS just an appendage to Siuan, functionally speaking. She is not, however, a motivating factor or integral part of Siuan that renders her essential to any discussion of the latter. For example, you could hardly discuss the characters of Perrin or Faile without including the other, but Leane is in no way significant in Siuan's character, except as just one more person with whom she has dealings. Egwene holds more importance in Siuan's character analysis than Leane.

With that out of the way...

They are not exactly bad or at fault, but simply in over their heads for the circumstances. In other times, Siuan or Elaida would have been perfectly acceptable and suitable as Amyrlin Seat, and might have been hailed as very good ones. However, they were not equal to what the end times in which they came to power demanded, and thus they had to go. Siuan is an Elaida who happened to get on the good side of the main characters, and who was removed before the crises really piled up. Had the order of their reigns been reversed, the story would have been little different. Instead it would have been Siuan seething over the hints she unearthed of what Elaida the Amyrlin was scheming with the royal line of Andor and their connection to the Dragon Reborn, and motivated by her resentment of the events of New Spring to tear Elaida down for the good of the Tower, only to see everything turn to dust in her hands as she tried to pull the strings that move the world which are now attached to broken or severed limbs.

Siuan pretty much deserved her fate, and is sort of representative of what is wrong with the Tower. She was fixated on the Black Ajah because of her terrifying experience with them early in her career. So much so, in fact, that she overlooked the glaringly obvious flaw in her reasoning that became the justification for her enemies. The charge that even her erstwhile supporters in Salidar could not dimiss out of hand was her secrecy and keeping the news of the Dragon's Rebirth to herself. Regardless of what one might perceive to be the rightful prerogative of the Amyrlin Seat, logically, keeping the news of a major sign of the apocalypse from the Hall of the Tower takes a good deal of nerve and arrogance. Why would she do that? Out of her petty ajah prejudices misleading her about the reaction of the Red Ajah and for fear of the Black Ajah. Except her experience in New Spring SHOULD have showed this woman lauded as intelligent and perceptive with great analytical insight that the Black Ajah TOO was trying to keep the Dragon a secret AND they already knew! It would be like a naval officer learning about the impending attack on Pearl Harbor and refusing to tell his superiors out of fear that the Japanese might learn where the US fleet is located. THEY KNOW! That is why they are currently attacking! Siuan was NOT keeping anything from the Shadow, since she herself reasoned out that they were ALREADY trying to kill the Dragon Reborn. All she was doing was preventing the sisters who served the Light from learning that they should be on the lookout for him and try to protect him. Instead, she plans to secretly bring him into the White Tower, where she has seen that the Black Ajah has the power to reach even the Amyrlin herself! How can she hope to protect him in the heart of the Black Ajah's power, when they and she are the only ones who know his importance, when her every move is scrutinized by the sisters who are politically savvy or interested in power (and a Darkfriend who does not fit the latter category would be a contradiction in terms), but she has no idea who they are. She seeks to bring him into the spotlight to conceal him from those who lurk in the shadows with ranged weaponry!

Later on, as events unfold around the rise of the Dragon Reborn, she becomes obsessed with drawing out the Black Ajah and finding them as the most important issue, completely neglecting her own political security and the fact that if she is pulled down, she can do nothing about Rand or the Black Ajah.

In both instances, her obsession with the Black Ajah and her willfully misguided plans for Rand, her error is in her assumption of the supremacy of the White Tower and her misunderstanding of the extent of her and its power. We see her trying to accomplish her Murandian plot by imposing it from the top down, having been raised in an oppressive society and then trained in a strict, ordered hierarchy and never imagining the complexities of the real world, where the lowest may act for themselves, and not according to set-in-stone traditions and unwritten rules of the plots and counter-plots within the Tower. Her isolated and parochial upbringing and education give her an excessive affection for, and an exaggerated view of the importance of the White Tower. She cannot truly grasp that the outside world may not wait on her purging the Black Ajah from her midst or that there are factors beyond her control. Moiraine herself identifies that last tendency when she thinks that Siuan cannot accept that "the world will burn" no matter how they scheme to protect it. Gareth Bryne essentially notes the same trait, however endearing he may find it, in tGS.

We see that her agenda and perception of the correct way to handle Rand differs in no particulars from Elaida's. She also sees having Aes Sedai minders on him as an absolute priority, and her willingness to let him go his own way at any point derives from her absolute trust in Moiraine's assertion that this is simply a technique to properly control him. She tosses out her best estimation of where he is as a ticket to the counsels of the Salidar leadership, and never mind the details of who will be given this knowledge or what they will do with it. Let a couple of Black sisters be sent to "run" Rand with a ton of warders between them, or a couple of arrogant women who might exacerbate his suspicion and hostility toward Aes Sedai, or anyone at all - so long as Siuan can get back in the councils of power. In her mind, anything can be fixed if you have sufficient Aes Sedai authority.

Siuan, like the Tower, overestimates her importance in the scheme of things, and labors under the delusion that she is more of a player than she truly is.