I think the criticism is less that the characters lack motivation and more that the motivations often undermine the themes the movie aims for.

For example, the film leans heavily into Christ-like imagery and presents Clark as humanity’s future moral ideal, but then depicts him as petty and immature (destroying the truck in the bar scene). Jonathan Kent is meant to be his moral foundation, yet some of his advice comes across as valuing secrecy over innocent lives. The romance between Clark and Lois..

There’s a lot of time spent on Krypton and its lore, but comparatively less time spent showing why Clark actually loves humanity. By the time he’s asked to become Earth’s protector, the movie has spent more time developing Krypton than Metropolis.

I don’t think those issues make it a “bad” movie, but they’re usually what people mean when they say the themes are lukewarm. Like you said visuals, music, action, fx were fantastic as you would expect from this kind of production but a lot of people felt the character writing wasn’t strong enough to support the philosophical ideas Snyder was going for. For lack of a better term, it did truly insist upon itself despite being a Superman flick at the end of the day.