IMO, the main problem is that this "efficiency" usually involves making assumptions that can't be altered, which achieves "efficiency" by eliminating choices normally available to the user. This is rarely done for the benefit of the user - rather, it just reduces the UI dev work, and more importantly, lets the vendor lock-in the option that's beneficial to them.
In fact, I've been present on UI design discussions for a certain SaaS product, and I quickly realized one of the main goals for that UI was to funnel the users towards a very specific workflow which, to be fair, reduced the potential for users to input wrong data or screw up the calculations, but more importantly, it put them on a very narrow path that was optimized to give results that were impressive, even if this came at the expense of accuracy - and it neatly reduced the amount of total UI and technical work, without making it obvious that the "golden path" is the only path.
It's one of those products I believe would deliver much greater value to the users if it was released as an Excel spreadsheet. In fact, it was actually competing with an Excel plugin - and all the nice web UI did was making things seem simpler, by dropping almost all useful functionality except that which happened to align with the story the sales folks were telling.
