I think that if syncing were as reliable as it should be, people wouldn't be asking for the sync button back.
However, Garmin says that in order to sync all you have to do is connect the device and it will sync automatically. Effectively, connecting the device is like pushing the sync button so it's not like it's completely gone. That being said, my vivosmart, vivofit, and forerunner 305 sync perfectly with Express.
I had issues upgrading maps on my GPS so a sync button would have been convenient that time. Instead, I had to disconnect and reconnect the GPS until it finally finished without erroring out.
You'll need to provide some details. How are you trying to sync? Just with Express? With a smartphone? Android or iOS? What happens when you try to sync?