Generally, this makes things super convenient for you. It takes a fraction of the effort and research to get a new language or feature going.
It also means that if you need something different from the defaults, it's another thing to learn how to modify a layer to suit your needs (not as simple as I had hoped). Also, if the package you want isn't available as a layer, you have to create your own custom layer for it. This gets you trying to think about organizing your packages, which while maybe useful, can be a big time sink too.
Using it for about 3 months now, it's quickly become my go-to editor. I'm very happy with the defaults provided by spacemacs layers, and haven't yet felt the need to modify them much. I did look into it once and got scared away. All the customizations I've made are in regard to Emacs defaults themselves, and fit nicely into my .spacemacs file.
Learning emacs and vim from decades old documentation is quite a journey. Keep an eye out for Stackoverflow answers. They tend to be the most up to date and relevant.