The original dump-bucket style scrubber has moving parts, makes noise, can splash, when the turf is exposed is smells, they're bulky, large, and expensive. You can buy them from Inland Aquatics and they pay a royalty to Adey I believe (one of the reasons they are expensive) so you can't really get them made anywhere else to my knowledge. Very early on in this thread there is a nice example of one someone built that makes a nice surge in their tank though, so they're not completely horrible, just much more complex and difficult to build.
If you don't go with a dump-style, then it's easier, but you still have a lot of horizontal space requirement. The screen needs to be double the size that it needs to be for a comparable vertical scrubber, so if you have a 100g tank you would normally need a double-lit vertical screen 100 sq in (L x W), for one lit on only one side it would need to be 200 sq in (L x W), and for horizontal you need 400 sq in (L x W). Then it needs 50% more light because you need to spread it out more to cover the area, and then you're still running it at a lower watt/sq in.
Horizontal non-dumping screens also have a tendency to 'channel', where the algae will grow thick and divert water around it, leaving pockets of stagnant water that will have nearly zero flow and no boundary layer flow to speak of, which all but eliminates filtration in those areas. Slanted screens have a little less of this, but it is still there.
These are all the issue of the past that really made algae scrubbers ineffective. That and poor lighting, that's a big one.
So if you're going to make a horizontal scrubber, the key is to at least try to incorporate some kind of surge device, perhaps a constant low flow to keep the algae covered and help keep the smell down and a surge box to dump at regular intervals, and over-light the snot out of it.
