I'm not very familiar with loopback but I try to show up some key differences:
You need to setup and configure each loopback project yourself.
the kiss backend is usable out of the box (selfhosted or as a service)
As far as I know loopback only does REST. In kissjs everything will be realtime.
In Loopback you have to create endpoints for every entity you want to have crud methods for. eg. you want to save Todos and Bookmarks, then you have to setup your loopback instance to handle that. With kissjs you don't have to do that. you can easily access any kind of collection from the client and only define access control. The data layer is more comparable to firebase/firestore
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