I wonder how the tests were done, especially that I work on Magento and have own thoughts comparing Shopify and other platforms. Magento is a big thing, packed into community version, Yes, its slow, but it can be tweaked. Btw its no.1 platform in usage currently wordlwide. So your guides is a bit skewed I think……. Magento is not absolutly pricey, I can set it for free on $5 VPS on Digital Ocean
BigCommerce is most easily compared to Shopify. Both platforms offer a similar experience when building an ecommerce platform. Like Shopify, BigCommerce offers a range of prices and packages tailored to different types of businesses. The platform is highly customizable if you are comfortable with some light coding. It is also possible to use themes and templates to build your website, but some of these will come at an additional cost.