What is the ruling on finding fault with food? Is this considered backbiting against the restaurant owner or chef, taking into account the various motivations behind such an action?

Finding fault with food is contrary to the Sunnah (definition), for the Messenger of Allah, peace and blessings be upon him, never found fault with food. If he desired it, he would eat it; otherwise, he would remain silent. If finding fault with food results in harming its maker, and the food was well-cooked but its taste did not appeal to the eater, then there is a concern that this might constitute forbidden backbiting (gheebah), because backbiting is mentioning a person with what they dislike. And criticizing food displeases its maker and could lead to customers being alienated.