My experience with the gentler flushes is that they really don't do enough. If that is what you are looking for, then I would recommend LiquiMoly, as it very well known and used alot by owners of German cars without issues.
You may wish to use the gentler flushes like LiquiMoly to do some cleaning, and then eventually use stronger flushes.
The flush I've found most effective is: BG EPR 109. It is likely the most effective engine flush as it is one of the few flushes that targets piston ring cleanliness (in addition to sludge/varnish). It has very different ingredients and odor than other flushes I've used. You can find it on Amazon: 4 cans sold together for about $45 total.