Lapis and Jasper have quite possibly the most realistic depiction of an abusive relationship ever shown on kid's TV. It's not even hidden - it's straight-up abusive - and is resolved in the end with violence.
Some scenes are extremely trippy and confusing, including an Uncle Grandpa crossover, Pearl dreaming of Rose turning into Greg before vomiting a slice of pizza at random, and bizarre colors and flashing lights, which may bother younger viewers.
The Kindergarten, especially the abandoned gem injectors (which resemble viruses) may upset and frighten some viewers.
Even Steven Universe is similar to Adventure Time and cartoon, this show contains serious issues/dark theme in Cartoon Network show. This could pushed the TV-PG.
Whilst not usually scary, Steven Universe can become very intense and mature for a kid's show.
Darker and deeper themes are explored in this show, including consent, abusive relationships, war, murder, genocide, depression, anxiety, grief, bereavement, and PTSD.
We find out very early on (2nd episode) that Steven's mother, Rose Quartz, essentially died to create Steven. This is brought up over and over again throughout the series. The effect this has had on the other gems is shown, especially Pearl who was hopelessly in love with Rose Quartz for millennia.
The entire climax of Bismuth is very intense.
Cat Fingers (6th episode of the show) is infamous for the part where Steven accidentally turns himself into a cat monster, with lots of body horror shown in explicit detail, especially for a kids show. Both children and even adults may be frightening by this scene. In fact, this episode is so unsettling that Estelle, the voice of Garnet, stated in an interview that this episode is "the least kid-friendly episode of season 1A", which says it all.
Frybo might frighten children.
The whole forced fusion concept is quite disturbing.
It should be noted that in non-US airings of this show, some of the less child-friendly content in this show (all categories) has been removed. (For example, UK airings have censored many episodes of the show).
It should be known that Homeworld is a very totalitarian society that immediately kills any gem that doesn't fit in without batting an eye. This is shown as a stark contrast to Earth, which is open and accepting of defects and differences, and embraces them.