So far, so good is used almost exclusively in the middle of whatever action or event is being described. See example from MW:
—used to say that something (such as a project or an activity) has proceeded well or without problems up to the present
"How's the work on your house going?" "There's a lot more to do, but so far, so good."
It is therefore said when the future results are unknown. Typically it has the connotation that the speaker expects the "good" results to continue. Exactly how sure the speaker can be interpreted based on the way it is spoken.
"Until now" is typically used when the "now" state is different from the past state. New information is known that wasn't before, or new products are available that weren't before. Here is a good source that has a lot of example sentences, including:
We declare that the demanded amount has not been transferred to the bank account until today, which is the final date for repayment.
We declare that the demanded amount has not been transferred to the bank account to date, which is the final date for repayment.
Therefore these two phrases are typically not interchangeable