That depends on how you build it. I run several of this type of scrubber and have zero problems with dying off at lower layers. That happens only when you wait too long between cleanings.
Also light source has a lot to do with it. With CFLs, layer die off happens faster. With LEDs (if close enough, spaced out enough, and at the right intensity) lower layer death is virtually non-existent unless you let it grow past 14 days.
Horizontal scrubbers have the same issue with shading. This is less of a problem if it is an actual dump-style that actuates and you get a lot of motion of the algae mat. But a static horizontal or slanted trough would have shading issues also, if not worse.
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication