Hi Lukas,
Thanks for posting in.
X theme is a framework, like a skeleton where you can start your development. It can work as is, or you can further customize it through coding.
The downside would be the time needed to familiarize X theme. Though, it’s better than doing from scratch every time you do developer a site.
The good side of developing from scratch is, it’s your own standard and coding style but again, too time consuming.
Thanks!