At what scale would you choose Magento over WooCommerce or Shopify?

I’m a front end dev with a potential client who has circa 20,000 products in inventory and an existing customer base in the hundreds of thousands. I’m wondering would they need a Magento commerce solution or would WooCommerce with scaleable cloud hosting perform for them? Also, generally interested in your thoughts on, at what scale does WooCommerce not meet requirements. Thanks