Then you can't use it across apps, though. Say for example you have a web front end, a mobile app, and a service receiving web hooks that all want to execute t

Then you can't use it across apps, though. Say for example you have a web front end, a mobile app, and a service receiving web hooks that all want to execute the same action. If you have that in a client layer, you'll be writing it 3 times. Moving logic to the client comes with pros, too (like being easier to work offline), but it definitely has downsides as well.