The mobile app had a notice about iCloud for a long time (I forget if it's gone now or not), but usually you're running into Obsidian trying to sync all of the files and rebuild it's internal cache when it opens instead of being able to do any background sync.

As for why is it better than Notion at that point, well, if you wanted to you could use a "faster" app like iA Writer on your phone, or anything that can open Markdown. That remains its biggest benefit, you're never locked in to files that are only on someone else's server.