At first I was a bit reluctant to try the relatively new Shopify Theme Kit that seems to now replace the previous Shopify theme app. So far I have been quite happy with the Shopify Theme app whenever I worked on a Shopify project, since it was a very convenient and good enough way of getting your theme synched and uploaded.
The Shopify Theme app usually crashed on me once in a while, but after updating to macOS Sierra, it started crashing quite frequently, so I finally gave Theme Kit a try.
I have to admit that I was positively surprised. It's very fast, more reliable and it does some things that I always wished the theme app would do, e.g. replacing files and down- or uploading specific files.
Even though the documentation isn’t perfect yet and could be better for some things, the learning curve actually isn't very steep and easy to master in the end. So far I am quite happy with it and it seems to be a good solution to synch your Shopify projects.
If you are working on a Shopify project and want to build your theme from scratch, I'd definitely recommend you to give it a go. It'll pay off shortly after and make your Shopify development much easier.