That is, if I create a Mobx or VueJS app, do I not need to wory about side effects using redux-saga or redux-observable?

My understanding of Mobx is that the answer is "No". And if you don't use a side effects library, you end up recreating something equivalent. For example, consider the following use case:

• a table component showing data fetched from a table
• you can also filter the table
• loading and filtering are AJAX operations
• the table always needs to show the latest AJAX result. For example, imagine filtering the table, navigating to another app route before the pending AJAX request completes, then coming back. Presumably, the filter gets reset, so the table needs to show the unfiltered results.