Agreed! If the corrugations are deep then the brush just sits on the ridges and doesn't scrub the valleys - which is where the algae is! If you try using the brush flat, you'll clean the top and sides of the corrugations, but leave the bottom of the valleys untouched!
The best (= least difficult) way to do it is to use one end of the brush and do one side of one valley, then use the other end of the brush and do the other side of the valley. As long as you pre-soak with pure water or maybe TFR, it's quite quick to do each valley, but there are lots of them, so it's VERY time-consuming.
The top photo shows how the bristles DON'T get into the valleys to scrub. The lower photo shows how using just one end of the brush does get into the valley.
Price it well - it'll take AGES and make your wrists ache.