There are so many, that each of them are becoming weak dealbreakers, because otherwise I would not read any comics anymore. Also, some of them are a problem only when it is too intense (eg. I don't like commentary-heavy comic but will not mind a bit).
What I never like:
Vertical format. I read some of them but there better be a very good reason that push me to read it. I really, really dislike reading in this format.
Exagerate sexual dimorphism. If characters of opposite sexes look like they are different species, I'll likely skip the story by default. Exagerate eye size difference is enough to completely put me off. In the same idea, partners looking like different species in m/m romance. I'll pass.
Stories where one can separate characters in good ones and bad ones. I see no interest and no relatability.
Redemption arcs that are clearly there either only to add content or for ideological/religious reasons. I generally find the trace of one or the other in most redemption arcs so they became a dealbreaker by default unless I have the proof they are there for a valid reason.
Glorified crime without any repercussion or explanation or opposition, especially criminal acts in a romance context.
Fourth wall breaking in non comedic context.