Vague, inconsistent and non-obvious guidelines with regards to themes and apps and sudden (and unannounced) changes in policy have bitten my company in the past. I wouldn't go as far as to say that Shopify are hostile to developers, and some are clearly very successful, but I now approach the Shopify platform by being constantly ready to write off any time invested in building for it.
To be fair, you can say precisely the same about building for Apple's App Store and the Google Play Store, and others. But Shopify is certainly equally capricious.
(Throwaway, because I still have apps and themes in the ecosystem.)
We also sweat over every API addition, often launching new features API first before we even put UI into the product to use the same features.
The developer platform is really core to us even if it sometimes doesn't strictly make economical sense to divert so much potential revenue to others. Yet, that's the long term game we are playing and it's working.
The reason is simple: Commerce is really different business to business. There is no way to build a single (good) product that solves all requirements.
Our philosophy is pretty simple: Shopify adds whatever most people need most of the time. This changes over time - towards the end of the last decade people sold mobile optimized themes. This was something we had to build into the platform because everyone needed that. Things that only some people need some of the time ( or similar variations on the theme ) should be supplied by the app store.
If you find inconsistencies between what's happening and what I'm saying here, please email me at tobi@shopify.com
The day before they rejected it, they added similar functionality to my own.
I think they were waiting for their own update to drop. I don't believe they 'stole my idea' but its still kinda shitty.
I understand and can relate to some of the hardships the sibling throwaway comment is referring to, but I feel you're going to face challenges whatever you do when building a business, and this is simply one that I'm comfortable with choosing.
An ecommerce platform that has very little support by developers but a ton of uptake by users is Magento. They desperately need functionality that is given away as free apps for shopify users.
Of course, Magento is a php platform so it's not nearly as sexy to developers (me included).