The series is incredibly violent in its action scenes, though there isn't much blood.

The blood of Cross! Sans is discolored, however, he is seen covered in it.

A some uses of "Hell", "Damn" and "Asshole"

Nightmare! Sans is seen giving X-Event! Chara the middle finger and constantly mocking, cursing, and physically hurting him despite being partners.

There is a scene where Fell Sans repeatedly tries to curse, some implied to be pretty explicit, but is all censored by Fresh Sans, so nothing is actually "said", only implied.

Underswap! Papyrus is seen smoking a cigarette in in Underverse 0.3.

In underverse xtra scene 2, asgoro is seen smoking a cigarette in undernovela.

Fight scenes may be fairly intense

The character Nightmare!Sans may be really scary for younger viewers.

The episode Underverse 0.6 may be the scariest to younger viewers due to it's imagery.