Sed here, (the author)

Image tag in Sundown is easier than Markdown:

    :s:img http://link-to-image.jpg  image alt tag

Or how about embedding a tweet:

    :twitter:embed  http://link-to-tweet

Or, youtube, user sign-up form and more...

You are absolutely right! There is a trade-off. Some tags are easier in Markdown (e.g. bold, italic, inline code and a few others). But you cannot extend markdown with your own custom elements can you?

Sundown by itself is not that useful. It gets really useful when you use it as a preprocessor for your own custom HTML tags.

Here I explain how you can create your own server-side HTML tags:

https://www.sunsed.com/manual/creating-tag

Again, Sundown is just a HTML preprocessor you can write all of the above tags like this:

    <s:youtube>
        http://youtube-link
    </s:youtube>