Indeed they are not. Unlike the Sedge Warbler for instance, which will often perch on top of the reeds, the common reed-warbler is always in the lower of middle level, making it way harder to see. This individual was only about two meters away from me. It stayed perched for minutes, I also made a video of it. Great moment.
I also have recently be able to photograph the Marsh Warbler, but those photo's did not turn out as good as this one did.
Cetti's warbler are even harder to see/photograph than common reed-warblers. I've heard them dozens of time, but seen them only about 5 times, and photographed it just once.
'De Biesbosch' has a growing population of Cetti's warbler. This time of year, you'll hear them everywhere (in the suitable habitat ofcourse). But you probably know this