Each of the steps in building up the tree of comments, likes, etc. is a a couple writes at most. If that fails, you're left in a safe state. You tell the user it failed and carry on.

Deleting a post and the comments on it is harder. First, you have a potentially unbounded amount of DB rows to delete across an unknown number of shards. Second, you now have to worry about the order in which you delete things, else you risk being unable to retry in case one of these deletes fails. Third, unlike the inserts and updates, you can't just give up partway, as that's not a safe state to end in.