There are a few conditions under which that type of merge fails, including when relational integrity is in place. Relational integrity is usually regarded as a "best practice" for data warehousing, so using this type of merge actually causes all sorts of issues. From a performance aspect, I have also found that it doesn't generally perform as well as an update/insert pair within a transaction.
I'm not a huge fan of SSIS; just because it is graphical doesn't mean it is "easier to debug" -- quite the reverse! It has some strengths, and used carefully it can be an incredible tool. Having cleaned up more than one SSIS spaghetti nightmare though, it's also quite dangerous in the hands of people who don't have a very structured careful approach to using it.