Mercedes recommends the citric acid flush to remove SCALE (lime) from the system. Not because it's "gentle" because it isn't. Take some oxidized aluminum pieces and throw in a vat of citric acid diluted to the strength that MB recommends and watch the fizz. The fizz is the acid attacking the aluminum and stripping away the oxide layer (for those familiar with physics, that's removing material). We use it at work to clean up oxidized aluminum for that very reason, it also requires a good neutralization bath (we use baking soda in water) to stop it oxidizing again later. They also suggest to use it only in dirty systems or vehicles with overheating issues.

If the car has clean coolant in it, it's been changed semi-regularly, and has been topped up with distilled water instead of tap water, there will be no scale in the system, or if there is it'll be a small enough amount to not bother with.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]