The tension the ownership brings between them! It is such a sore point, Mensah acknowledged that multiple times in ES (I understand why you don't trust me, I know where I went wrong) and the tension is still there in PD (it's 80% certainty that Mensah would be sad if it died. it's only 80%, about Mensah! Who has saved it in turn many times, stood in front of danger for it, is targeted by assassins, drops everything any time it needs help to get to it, has secret special codewords with. That Mensah! And it's only 80% certain she would be sad if it died and not like incredibly heartbroken)
The tension the ownership also puts between Mensah and her family (Amena's horror that her mother owns SecUnit! and that when no other family members around, Amena owns it by default!)
Hell even Mensah's job is harder for it. She legally owns a person. She legally owns a dangerous weapon. She has final say and control of its movements and presence in Corporation Rim and Preservation. She could, at any moment, change her mind and demand it come back and even without a GovMod - the broader system it lives in means it would be safest for it to return (as to if it actually would...)
There's tension with her fellow councillors, with journalists, with station security, with corporate companies. And it's all because Mensah (Madame President Dr Mensah) legally owns a (rogue) secunit!
SecUnit's in a place where it can joke about its owned status (calling itself a pet bot, pet security consultant, domestic fauna references etc) but it also has moments of such bitterness when it brings it up (because my owner said so, internal narration about Amena's horror at owning it, pointed conversations with Farail - technically, despite their care for it - a slave in the CR and essentially a slave on Preservation and it doesn't want her ordering it about.)
Mensah, Pin-Lee and Bharadwaj are working on changing bot rights and the guardianship requirement entirely, especially around autonomy. Not just for SecUnit, but for all bots. They're looking for work arounds to let it be as free as possible - job contracts, refugee status, encouraging independence and giving it a safe place to return to.
But they know it's going to take decades. The documentary and legal cases in Preservation are just the beginning. It's going to take time and hardwork. They may never see it in their lifetimes, they may never be able to remove this chain between the two of them that weighs down their trust of each other.
It's such a perfect example of If you love it you let it go.
(I think ART recognises this as important too, as at end of AC its the one encouraging SecUnit to leave, and while it invited SecUnit on missions with it, it's shown no sign of any jealous or upset that it also lives on Preservation and moves between the families.)