This has been a tricky topic for a while now, and is generally pretty poorly explained to people interested in just what a symbiote is and how it fits into the world and worldviews of MAC characters. Hopefully this will offer some clarification.
The Borders of Sentience
An auracore symbiote, despite enhancing the functionality of a mecha and occasionally possessing rudimentary personality, is not sentient. Despite being engineered for a specific purpose, a symbiote does not represent the achievement of artificial intelligence. Part of the reason AI wasn't truly developed was that magic is often the substitute. For example, it's easier to bind an elemental into a mech and animate it like a golem than to develop artificial intelligence.
Some elemental spirits are more advanced than others, yes. But auracores are engineered for a specific purpose instead of grabbed from the plane of air or something as-is. The fact that it took centuries for a world very familiar with elementals to learn how to make auracores should tell you just how much effort had to go into it. There's little doubt that someday there'd be sentient auracores and then things would be wacky.
It's an astral elemental that's very limited in autonomy and flexibility simply because it was designed to do one thing and nothing else. It interfaces with the mecha. That's it. It's the spirit in the golem.
Teks have to walk a fine line between having the advantages of a pseudo-copilot and the disadvantages of having that copilot disagree. There are advantages to it, but the fact that you can lose control is a terrible thing and many pilots wouldn't want to deal with it.
A symbiote is very nearly a full co-pilot, except that it lacks the autonomy and intuition to function on its own the way its pilot can. It's only about as intelligent and autonomous as a dog, despite its ability to use the mecha's computer.
A dog isn't sentient. A dog knows friend from foe. A dog often knows its master form other friends. A dog also generally has an idea of what is expected of it. You can see this with police dogs. It knows whether it's supposed to bite enemies, sniff for drugs, chase and point, etc.
But you can't explain to a dog why these things are important. You can't explain to a dog why it's doing it. The dog will not understand, and it isn't the dog's job to understand.
Dogs even have very complex and variable personalities. Some are erratic and some are dangerous no matter how well you treat them. You can manage these dogs and deal with them, but you can't reason with them. Because a dog is not a person no matter how many similar functions it can perform.
This is why it takes a special kind of pilot to work with an auracore symbiote, just as it requires a special kind of police officer to handle a K9 unit. You need a human in charge so that things stay under control. A normal officer can't do that, and a normal mecha pilot can't do it with an auracore. You need a K9 officer, and you need a Tek pilot. Otherwise you'll have things performing their function without any control over them, animals running around barking randomly when they smell cocaine, and mecha blowing random things up simply because they were designed to blow things up.
Nanashii is a productive example. Nanashii identifies as female. She loves blowing things up. It's in her nature to blow things up. She can do it, and she's programmed to do it, and she has weapons and when her pilot tells her so she's even got enemies. She would blow everything up if her pilot weren't there to direct her, simply because blowing things up is what she's designed to do. It makes her unruly, but it also makes her damn good at blowing things up if you can point her in the right direction.
There are people who interact with these symbiotes on a semi-regular basis who do not hail them as the awe-inspiring product of MAC's most brilliant minds. Because of the line they tread between sentient and non-sentient, not to mention the fact that they're lifeforms engineered by scientists to serve them, some view them as abominations and perversions of the natural order.
A further issue for many observers is that symbiotes are not generally aware of the limitations to their autonomy. 99.9999% of mecha don't have high enough EGO to operate on their own. The ones that do are still only performing the function for which they were programmed. It just occurs to them at one point to do it (in the short term) on their own. Little to no personal initiative. They just do what's in their nature to do.
The auracore can do things. It's able to. But it doesn't have the mental flexibility to decide what needs to be done when and why. A symbiote may feel quite motivated to destroy enemies, but this is because that's all it knows, all it's been programmed to do, and it requires the input of a pilot to tell it what constitutes an enemy and how the enemy should be destroyed. Whether auracore symbiotes think they're sentient and self-governing or not... they aren't. This is a source of some confusion and consternation for observers hoping to figure out whether the symbiote is self-aware and self-governing.
Symbiotes are very nearly astral elementals, but do not occur naturally and are therefore sometimes considered outside the natural order of life and death. It is unclear at this point what will be done with these creatures upon their destruction, or even what constitutes their destruction. Reshtahan, Tunarian and Ashuran scholars are still in debate over the issue.