Well for one thing they seem to have have quite a comprehensive tax system which they keep updated. Something I realised could be valuable when dealing with a Californian client on a different Ecom platform.
Another one is a reasonably powerful but client friendly interface, which sounds easy but almost all other platforms fail in some way.
Lastly it isn't totally bad for headless ecommerce, at least if you dont look at the accounts system too hard.
Shopify has become the default platform for eCommerce for people who have their own marketing channels (let's be honest, the main draw of selling on Amazon is they are a default shopping search engine). I've noticed a lot of "influencers" with strong brands have started shopify stores to sell clothing lines etc. If you can get eyeballs outside the Amazon search results, shopify is probably where you're directing them.
That said, they solve some difficult problems and no one else comes close at the moment.