When working with large data sets incremental loads will always be better than full table loads correct? Are there any downsides? And why not have that selected by default when it’s available for certain fields?

Incremental is better for large datasets. After the first sync you’re going only to bring the new records. Sometimes you’re limited to and need to capture deletes or updates that is not possible with incremental method. About the sync method hierarchy there is an issue requesting to change the default.