Everyone sees this as a feature, I think it's an anti-feature. Personally I hate the idea that there is software on my phone acting against my will to delete content that I have been sent. Software running on your device should always be working for you, not other people.
People should think twice before sending a message or picture and stand up for what they do, not invoke malware that they enforced on my device (by only using WhatsApp or similar message apps instead of SMS etc.). They shouldn't be allowed to delete data on my device, even if they "created" it. Ultimately they seem to have more power over my portable palmtop computer than I have, since I cannot prevent them from revoking messages.
And if you do change your mind about something you did, you should man up and take responsibility for your actions. At most, there should be an option to send a "hide" (but still make retrievable at will) a message in case of typo or other similar error. And obviously cancel a send if the message wasn't delivered yet (messaging service companies have all the right to do whatever they want with their servers, as long as they don't touch what is already on my phone).
All unsend does is pose any hacker/malicious user of a messaging platform in an unfair position, put people under the wrong assumption that anything done on the Internet can easily be undone, and potentially delete proof of legally binding contracts or communications. I deem this unacceptable.
Edit: if your opinion is the same, different, opposite, you wanna add something or whatever please do reply, this is a topic I've never seen discussed and I'm very interested in hearing comments.
Edit 2: apparently you can edit on archived threads.