I find Sketch much more valuable when designing for native mobile applications for a variety of use cases (wireframing, user flows, cheap "interactive" prototyping, and asset creation...the Android Asset generator as well as Sketch Measure have been godsends). I've had a lot of success designing for both iOS and Android apps. However, I generally have to break things down (much more that I would if I were working in Illustrator) by specific workflows, controller views, etc... I also can't have the extremely complex notes/scratch/meta layers that I would normally have in a messy Illustrator document.

Honestly, I think that's more of a feature and not a bug. I find it much more useful having actual conversations w/ my team instead of trying to capture everything in one document by myself to later 'present'. There are better tools for facilitating conversation than outlining everything in one document.

That being said, performance is an issue and I still can't figure out how to successfully use it for web apps. For native mobile platforms, it has vastly expedited my workflow in a multitude of ways. Like many people have said, I would still use Illustrator for pure vector illustration but have generally used Sketch for my UI workflow.