A heat exchanger needs some length to enable efficient energy transfer. Water going down a vertical drain tends to cling to the sides and if the heat exchanger is there, it can do its job. It isn't anywhere near as efficient on the horizontal, since you only have flow in a small surface.
It's not the length per-se, it's the total surface area.
But yes, they do need to be installed vertically- horizontally the percentage of the surface area of the pipe in contact with the water drops to under 25%, with a corresponding loss of heat-transfer efficiency. A 3 footer with a 4 inch drain can do remarkably well- on the order of 45-50% of the heat is recovered at shower flow rates. Even 20" x 4" heat exchangers return a third or so, and it can often fit in a crawlspace.
See figure 2a (lower left) on this document for the performance of 4" bores of various lengths & flows:
http://www.gfxtechnology.com/EFF.pdf
Those curves are modeled on decade old GFX designs too- they're likely better today. PowerPipe's design seems in a couple of ways inherently better than the current GFX, and will likely out perform the GFX stuff slightly, but all manufacturers' designs are in a constant state of flux, squeaking out a bit more with each iterative release- can't say for sure year to year who will be a nose & a hair ahead. Canadian testers believe the squareness of the cross section of the potable water coil is the key distinction that makes for better heat transfer performance, since the squarer it is, the greater percentage of contact with the copper drain core, and they've ALL gotten squarer since 20 years ago.
I guess there's not much you can do about showers in basements, slab-on grade first-floors, etc., unless you want to dig a pit to accommodate. But it doesn't take much of a pit to install a 2-footer (if anybody still makes 'em that small- maybe Retherm? The all seem to have 30-36" versions.)
Any installation will work somewhat better if you insulate it with an R2-R8 wrap afterward, particularly if it's in an unconditioned crawlspace or something. (A double layer of bubble pack held in place with zip ties would do for most.)
Clearly it helps if the hot water heater is already located next to the main drain exiting the house though. A lot of issues can make retrofits less than ideal, but for some it's clearly the best, cheapest, most-effective solution for endless showers short of going tankless (for a lot more money.)
For greenies it's a way of adding another 10-12% to their solar fraction, or another 0.1-0.12 to their as-used EF of any fossil fired or electric hot water heater, etc. (There are plenty of LESS cost effective purchased efficiency options out there than a drainwater heat recovery heat exchanger.)
OTOH is could be just viewed as an enabler of wasting more water (if not more energy) with endless showers.