Asides from the general warning against building a (long-term) business on someone else's platform, I'd caution against developing for Shopify in particular.

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.)