It's like saying drinking consistently throughout the day is dangerous without specifying whether we're talking about bourbon or water.
That key variable seems to matter more than the format.
For example: how do you think a person would feel if they watched 30 minutes straight of "short form video" of kittens playing with each other as opposed to a person who watches 30 minutes of people telling them their political opponents want them to die.
Somehow I think these two scenarios would have very different "mental health" impacts. As with anything, it comes down to what people choose to consume, not how they consume it.