Thank you Min, it was extremely helpful, I can't choose you as best answer because you commented instead of answering, though.

"To be capable of" means that the subject has the ability to perform the action. For example, "This doctor is capable of performing brain surgery, but is incapable of bringing the dead back to life." On the other hand "to be able to" means that the subject not only has the ability to do the action, but also has the necessary resources to complete it in the given context. "The doctor is capable of performing brain surgery, but he is unable to do so for this patient right now because the doctor is already booked for the day." In general, if you're saying incapable, it means that no matter the situation, the action cannot be done, but if you are unable, you mean the action cannot be done given the situation. Everything one is able to do, one is capable of doing, but not everything one is capable of doing, one is able to do. As for that picture, "They are incapable" is correct for a couple of reasons. First, the part that follows before implies that the current robots do not have the ability to connect on an emotional level under any situation. Secondly, the empty box is followed by the preposition "of". The verb "unable" is usually followed by the preposition "to" whereas "incapable" is usually followed by "of". Hope this helps.

I'm not sure if this is correct but I've always been taught that unable is more of a temporary state, where incapable is more permanent. Like, if a robot it unable to do something it is most likely something that is fixable or it could be taught, whereas if it is incapable of doing something, it is because it is impossible for it to do, or be taught. Hope it made sense