My husband and I downloaded Forza Horizon on Xbox One (2 separate consoles btw). We went in to play the game and after going into it, found our stuff from our 360 saved files were not there. We realized that we had saved them to the hard drive on 360 rather than the cloud. So, today we went in and moved his saved game and my saved game to the cloud. However, when I went into the game on the One, it did not sync with the cloud file from 360. I tried to contact support and the guy was clueless about my issue. He had me doing things that of course did not help. I finally uninstalled and reinstalled the game on my One, but it still will not sync with my cloud storage from 360. When I went into the game, it is still showing me as a beginner. Any suggestions? Please. 
Have you installed all your 360 DLC on your One? Go to Games/Apps on your One, find Horizon and select Manage. Any DLC should show up here. If you had a lot of DLC on your 360 you’ll need this installed on your One for the saves to sync.
I made sure all of my dlc was downloaded and went back in this morning. It popped up for a couple of seconds saying that it was syncing, but when I went into my game, it still showed me as a beginner. My stats from my saved game on 360 (where I had finished it all pretty much) are still not there.
You could be having the same server problems we’ve all been having. Reports suggest that a full system reset could be a temporary fix - hold the white button on your XBO for about ten seconds, then turn it off at the power point for about five or ten minutes, then restart.
With any luck a bit of patience might work as the traffic caused by GWG and BC subsides.
I tried that, too. My husband went in to his this morning after I made sure he had everything downloaded like you said and his worked first try. I uninstalled and reinstalled everything on mine again, but it still did not sync.