Sudden rearrangement of content is disturbing
Dynamically updating data while user is looking at it may (depending on type of data) be disturbing to the user workflow - for example, if you're reading a sentence and it changes while you're doing it, it's unwanted.
The same applies for any content changes that will re-flow or reorder other content. Appending something at the bottom may be fine depending on the scrolling behavior, but still dangerous; but inserting or updating some content in the middle (e.g. discussion threads) will cause other content - likely including some sentence that you're reading at the moment - to be moved away from where it was. This is generally bad UX, as the user is suddenly forced away from what they were doing and they'll need to find the content once again to move on with reading.
Good examples of continuous automatic content updates tend to be either in-place updates (a counter or graph with changing values but fixed size and location) or append-only content (chat, logs) if the speed of new information is limited and expected by the user.