So this movie is probably a lot better than what you might have heard. But some parents may have some issues with the themes and things portrayed in here in general. It may be a bit too much for their children to watch. There are some adult situations here (as in life happens and the "brutality of it", without getting explicit), some dramatic moments that may stay in ones mind.
Having said that, you could say, that this is challenging and if you really think about it, animated movies in the past that are considered classics, had some horrible moments in them (without spoiling the moments, just two examples: Bambi and Lion King). Even the Last Unicorn had some dark themes in it. So maybe we try to overprotect children, where they don't need to be protected? It's a fair question though, but the movie handles it very discreetly and to the point.