While not an open source platform, there’s a lot that is possible using Wix, with hundreds of API’s and lot’s more features becoming more open.
Just last week we announced a new feature where you can swap out Wix app pages with your own custom versions - Replace standard Wix app pages with your own custom version
There’s also a wide range of enterprise businesses using Wix which shows what is capable and how Wix/Wix Studio can serve these sorts of clients too - https://www.wix.com/studio/enterprise