[chjj]

I'll keep this as short as possible, but that's not easy.

Speed: It's relatively slow (but fortunately faster than markdown.js at the very least).

Quality: It carries with it a bunch of nonsensical quirks. Some of these quirks are inherited from the original markdown, some are unique to showdown. (I feel like I could write a treatise on list behavior alone, and how ridiculously showdown and markdown.pl handle them). It's not what people are accustomed to/find intuitive today. Here's a few problems (with showdown, as well as other engines) presented in a more tangible way: https://github.com/chjj/marked/blob/master/doc/broken.md

Code quality and maintenance: Showdown is a line-for-line port of Markdown.pl. As you can guess, this is not ideal, especially when you consider that the original markdown was written extremely awkwardly - it's essentially a pile of regexes doing global replaces on a single string to produce the output. This is _extremely_ confusing and messy to those who are not familiar with a code, and the original author of showdown, as far as I know, no longer maintains the project.