Despite One Piece being a fairly light hearted series, and the violence not being too extreme throughout, there are still some scenes which can be disturbing throughout the series. These however are normally not presented in an overly violent matter but it doesn't take away from the impact.
Many of the backstories throughout are disturbing, emotional, upsetting, and intense.
There are many, MANY emotional and sad scenes throughout the series. Many minor and major characters died in very tragic events, and some characters have devastating emotional breakdowns that result from their parents/siblings/friends dying. Although the series is lighthearted and happy-go-lucky, it has a very emotionally intense tone, but the humorous scenes can help recover from the emotional/upsetting stuff.
There are many scenes of tension including perilous situations, destruction, national disaster, war, etc.
Different forms of racism are occasionally present in the show, especially the land of wano arc.
The main cast, while pirates, only resort to violence if there's no other alternative, and there are moments when the more aggressive characters are shown that violence will only make matters worse.
While the design of this anime looks like a cartoon (what with it being very colorful, bright, cartoonish and lighthearted), but don't let that fool you, because it has a lot of very dark, mature and extremely emotional stuff.
Out of the big 3 anime of Shonen Jump (others being Naruto and Bleach), One Piece is the longest running and lighthearted series, with 1100+ episodes (due to long arcs).
There's lots of very mature themes in the series, though the cartoonish aesthetic and mostly light-hearted mood of the series balance them out.
As the series progresses, it gets darker, intense, emotional and exciting in much later on.
Luffy and friends get involved in a lot of problems during the adventure to find a legendary treasure, "One Piece".
Mild/Severe for neuordivergent viewers.
The fellas who have Devil Fruit powers are also unable to swim. Which can be a risk for many like Luffy, Buggy, et al.
This show can get fairly violent at times. There are several instances of slapstick humor, but others bloody violence can get intense for some viewers. Characters use swords, axes, knives, cannon, saws, guns, and explosives.
For older tweens (11-13) and up.
Moderate overall. Although sometimes it can get severe.
Some villains like Kuro, Bluejam, Hody Jones, Doflamingo and the Celestial Dragons are complete monsters who are devoid of empathy and sadistically kill and torment anyone in their way.
Though nothing pops out or scares you, some scenes in which people are on the brink of death or in an intense situation might scare some viewers. The lighthearted and very funny scenes can help recover from the intense stuff.
Lots of scenes where main characters come very close to death.
Emotional and intense scenes frequently occur throughout the show. And many of the characters' backstories are quite tragic.
This is a surprisingly dark and emotional anime overall, even if it's for the most part lighthearted and happy-go-lucky.