Apples and oranges. Slack's story is more about their service and 3rd party integrations, less so their client apps. A messaging client needs far fewer client

Apples and oranges. Slack's story is more about their service and 3rd party integrations, less so their client apps. A messaging client needs far fewer client features than a service like Evernote, and can get away with a simpler development environment. A web frame in a box works for Slack, but not for Evernote. Evernote's client from a feature and polish perspective is as important as their service. The service need to store and sync (where they also have problems), but the client has to cover a multitude of other features. Cutting corners on the client layer to ease development has had an obvious impact on the UX.