Azure File Sync is a really useful technology especially if you have file servers with lots of files which are not used very often. With cloud tiering enabled you can reduce drives with terabytes of data down to gigabytes.
But there is one thing to watch out for if you have hundreds of thousands or millions of files.
If you have for example half a million files and find that a 30GB drive is filling up even though you have cloud tiering enabled at say 80% then you've run into the same issue that I did.
I spent weeks working with Microsoft to try and figure out why it looked like cloud tiering wasn't working. I could see all the files were offline with APLO attributes but still the disks were full. I'd picked this server up from someone else and hadn't noticed how the disks had been configured.
They'd been formatted with 64k allocation unit size. This meant with half a million files they were using over 30GB of space. Reformatted to 4k block size and now my disks are less than 20% full.