I’ve seen enough data loss when the ‘magic’ of iCloud and Google Drive failed to ‘just work’ to be a firm believer in giving users control over their own data.
I can see what you’re saying, but I’ve been using Scrivener iOS for a while now, and yes, the manual syncing did bother me for a while (about ten minutes, give or take). However, in practice it isn’t really that big a deal.
The app does a pretty good job of reminding you when a sync is needed: you start up and it checks Dropbox to see if you’ve got any changes. If you have, then you sync. You also get a little symbol on the icon to remind you when you have changes to upload. In between that, you can forget it’s there. I don’t save periodically, so it doesn’t really interrupt my train of thought; when I’m done writing for an hour then one extra step isn’t really going to make any difference to my creativity. If it was intrusive then I would have a real problem with it; I just didn’t think it was.