There are scenarios where sympathetic characters are in peril.
Very upfront depictions of verbal and physical abuse from a maternal figure. Can be quite disturbing at times to young viewers.
"Promise" and "Remember" (season three in general, really) are very emotionally heavy episodes that can be very upsetting to some people.
Throughout the series there are themes of abuse, abandonment, heartbreak and self-destruction. The show is mostly lighthearted and funny, which evens it out the majority of the time, but it can be heavy for anyone of any age who is sensitive to that kind of stuff.
Shadow Weaver's treatment of Catra and Adora may disturb some.