I wouldn’t call the process particularly difficult. We review for things that can risks causing clashes with other extensions, e.g. global variables or files outside your extension’s folder, for Ruby errors that make the extension unusable, and for extensions breaking the undo stack (as this can lead to data loss). In my view, every extension published anywhere should really fulfil these basic requirements. Opting for another website for the reason that your extension is broken isn’t anything I’d recommend. We also make recommendations on design, but those are optional to follow.
That said, there are other reasons to publish elsewhere, and there are issues with Extension Warehouse.
The sign up process can unfortunately take quite a long time. We are also aware of the technical issues like in-app purchases being listed as free, but sadly Extension Warehouse hasn’t gotten the resources to be updated as we’d wished.
You may also want full control of your intellectual property and the full user experience.
SketchUcation is also older, and many old-time developers who have their full portfolio over there has stuck to it.