Planny uses a special iCloud sync to sync between Watch and Phone, not the standard bluetooth sync. This enables more bandwidth and the transfer of more data and also enables Planny without iPhone over LTE.
iCloud Sync requires Planny Pro, so to sync your data you need to purchase on iPhone, iPad or Mac.
Sync activation issues
In some cases (mostly on older watches or for users with lots of data) sync activation takes very much time. This changes with the brand new Apple Watch App in Planny 2.1.
Complications don't automatically update, because this isn't possible with current technology provided by Apple.