Our Playlist Routing App has a similar Out of Office feature. While I don't know exactly how Zendesk's app is implemented, I can say that there are API limitations preventing us from designing a scalable and robust solution. We get a lot of clients reporting inconsistencies too.

Our biggest challenge is with bulk updates. They are delayed and not guaranteed. We'd have to poll job statuses for UpdateConflict errors and retry failed updates. Polling counts against your rate limits, and it's slow. By the time the app retries, it's possible for an agent to be back online already. Polling and retrying is inefficient, defeating the purpose of bulk updating to minimize API requests, so our solution doesn't even bother retrying. Ideally, there is a synchronous bulk update option that we can use to guarantees an update every time.

Another fundamental issue is that there isn't an API to query for specific tickets using fresh data. Search API is not reliable due to the time required (1 - 5 minutes) to index records for search. We don't have an efficient way to get a full list of the agent's non-closed tickets at scale.

Once these API limitations are addressed, we could make dramatic improvements to our version of Out of Office.