@momothereal That’s not really their own “implementation,” but rather the addition of the Sponge API into the vanilla server software(same approach Bukkit took). What’s wrong with this? Well, let’s just use Bukkit as an example of what could go wrong.
Also note that it is now possible to download the latest successful 1.11 build on the website, under the “Future release” section. Please backup your world and submit an issues to the pull request. Thanks! :)