As convenient as it is, there are a few caveats and it isn't a silver bullet. The automatic feature will only kick in if there are no other compactions scheduled. Also, it is going to be single-threaded by default so it will take a while to get through all the sstables on dense nodes.
In contrast, you'll have a bit more control if you manually upgrade the sstables. For example, you can schedule the upgrade during low traffic periods so reads are not competing with compactions for IO. Cheers! >
