For both, it seems like I read messages in random order and then have to guess at the general gist. There's seems like no easy way to get an accurate timeline of things.
I think the problem is made worse when people forward (or retweet) as part of a conversation. There's often forwards of other email chains included as a reply with or without unique new messages being added.
Sometimes people are left off part of the CC chain for some messages, so there are gaps in the flow.
The problem is compounded when there are multiple email threads about the same thing, often because someone replied to an old version that turned into a new conversation. Or they read just the first sentence in the original email and replied about that in the newest follow-up in a 100 email chain.
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As an off topic side note, text chat programs are the ultimate form of communication. It has the fewest flaws of all mediums. It's linear in a single timeline that everybody has (and has the history to). It's pretty hard to mess up the history of what happened or who said what.
I enjoy it when people get frustrated about text chat. It's almost always because they can't dominate the conversation by talking over people or "win by talking loudest" which makes them frustrated and uncomfortable.