During the Mass Effect games, Commander Shepard of the Alliance Navy will meet, hire and fight alongside many talented soldiers, biotics and tech experts while taking on the galaxy's deadliest foes. Shepard needs trustworthy and skilled crewmates with very specific qualifications and experience, and the Asari justicar, Samara, is presented as a powerful ally for Shepard's mission. However, she might not be a good fit after all.
There is no doubt that Samara is good at what she does. She is a justicar, a wandering warrior-monk who dispenses brutal justice according to the arcane justicar Code. These vigilantes are essential for rooting out evil within Asari space, but this makes Samara an awkward fit for the suicide mission against the Collectors.
Since their homeworld, Thessia, is positively humming with element zero, all Asari are natural-born biotics. Samara is among the strongest of them, and she has honed her biotic talents for a particular role: hunting and assassinating criminals, violent outlaws, terrorists and other dangerous elements within Asari space. By contrast, Asari commandos, legendary for their combat prowess, train their biotic powers to handle a wider variety of foes, such as military opponents. Ideally, Commander Shepard would have hired an elite Asari commando for the Normandy SR-2's crew, not a justicar. Despite their prowess, justicars are a bit too narrow for Shepard's needs.
While Samara's personality was ultimately a good fit for life aboard the Normandy, there was no guarantee of that beforehand. As a justicar, she's used to primarily staying in Asari space, where her role as justicar is better understood. In fact, a traffic officer on Ilium was deeply concerned that Samara would cause an incident before long, as her strict adherence to her code would eventually cause conflict with Ilium's laws and procedures. On top of that, Samara is certainly not used to fighting in a coordinated team since justicars often travel alone, living an ascetic lifestyle while hunting wrongdoers. Contrast this with Garrus Vakarian, who loves fighting in elite squads, or Jacob Taylor, who is quite used to teamwork from his Alliance days.
Samara can still be a valuable asset. She's a devastating ally in combat, and her formidable biotic powers make her an ideal candidate to create a shield for Shepard's fireteam while navigating the seeker swarms aboard the Collector base. Other than that, though, Samara's skills and experience don't seem particularly beneficial among a foe on the scale of the Collectors. There is no way Samara or any other justicar has ever seen or fought a foe anything like the monstrous Collector race in Asari space. Samara is used to facing serial killers, terrorists and corrupt officials, not monsters from outer space, and it's unlikely that her Code would have any precedent for this. It's true that Samara swore an oath to help remedy this and follow Shepard's orders more easily, but Shepard didn't necessarily know that the oath was an option. Shepard was already concerned about the Code conflicting with their orders by the time Samara mentioned the oath.
By contrast, an Asari Commando or Spectre would have been a more ideal candidate for a biotic specialist in Shepard's suicide mission crew since such fighters are far more flexible than justicars and are used to fighting all sorts of non-Asari enemies. Spectres will do anything to get the job done, unlike the Code-bound justicars, and commandos are used to dangerous, exotic missions where survival and resourcefulness are key. If the Citadel council and the Cerberus organization weren't at odds, Shepard might have even convinced an Asari Spectre to join the team, or at least one of the best Asari commandos (if not a small team of them). Asari commandos, in particular, are used to fighting with a team to get the job done, and that's just what Shepard needs. A Spectre might have experience with teamwork, depending on the individual, but a justicar probably will not. Samara is strong, but her strength is the wrong strength for a mission against the Collectors.