"Is there some kind of catch I'm missing?"

Yes. Shopify is a platform that locks you in. You don't have a lot of flexibility compared to say self hosting with your own custom app. Of course there is a benefit to that but there is a cost as well. So it depends on what you want.

If I had a 500K revenue business, I will go custom. The reason: I want 100% control on how things are done because my online business IS my core product.