Welcome to the community and thanks for your thoughtful, detailed question! 
Building a Slack-like team communication app is a big but exciting challenge. Here’s a concise summary based on what’s available in the data sources and community discussions:
It seems likely that using an established chat SDK/API (like MirrorFly, Sendbird, or Rocket.Chat) will save you significant time and complexity, especially for features like real-time messaging, presence, and file sharing. These platforms offer robust APIs, white-labeling, and built-in support for group/1:1 chat, media, and notifications, which can be much harder to build and maintain from scratch. For scalability and real-time sync, many developers recommend a stack using Node.js with WebSockets, or leveraging managed services like Firebase for real-time data and device sync. Handling low latency and real-time updates typically involves WebSockets or similar persistent connections, and you’ll want to ensure your backend and database can handle concurrent connections and message throughput. Security-wise, focus on end-to-end encryption, secure authentication, and proper access controls—SDKs often provide these out of the box, but if you roll your own, you’ll need to implement them carefully.
Biggest challenges include: ensuring reliable message delivery and sync across devices, handling offline/online transitions, scaling infrastructure for thousands of users, and providing a smooth UX for file/media sharing and search. Community members who’ve tried building from scratch often hit roadblocks with message sync, group logic, and offline support, and found SDKs like MirrorFly helpful for rapid prototyping and production readiness. For voice/video, consider modular integration (e.g., WebRTC) once core messaging is stable. For more on real-world experiences and SDK comparisons, see this discussion and this post.
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