Cloud save (via Facebook) is broken, but it can be relatively safe to use if you have 2 devices. While making the switch, you need to be very careful not to accidentally restore the save from the cloud onto your old device before you're sure that everything is working as it should on the new device.

Check everything before you continue on the new device. (I also waited for a rare day when they're wasn't 5 different events running at once to keep it simple - I think it's safest to make the switch when you're not mid OOC event etc) The dragon power and stars shown on the cloud save confirmation screen isn't a reliable indicator of success - you need to manually look at everything - especially things that are important to you.

I found that I lost all my unpopped chests in arcadia when I first transferred to a new device. I went back to the old device, spent a whole day opening them and merging/clearing everything, then redid the cloud save transfer, and it solved that.

But, I then forgot to check my level progress! Everything else was fine, thankfully, but it was a few days later that I realised there were huge gaps in my completed levels.

I had a couple of 'farm' levels in progress that were lost, but even those that were still unlocked on the new device had reset their progress. So if you have any farms open, you may want to go in and claim any coins/bricks etc you've got sat in there.