So 18 months ago, when I got the 159, there was this the same problem of crazy wipers.
Solution that worked for me was to really, really, clean the screen. Twice, at least, over with Turtle Wax Intensive Cream Glass Polish - fairly abrasive to cut through the grime - then really careful to keep any Aqua Wax off the screen, and a good polish with ordinary Glass Cleaner after every wash.
Took a few cleans to settle down, but now I don't really need to use the Glass Cleaner at all. That said, you're right, the adjustment switch doesn't make much difference - some, but not particularly predictable.
And also, almost any other car's auto wipe is more predictable and adjustable that the 159. We had a Volvo XC90 for 7 years and its auto-wipers worked just fine, all the time, without any particular attention.