Flexamat is the solution to a common landfill drainage problem - erosion. It is critical that erosion doesn't compromise the landfill's cap. Once a landfill is closed the site has to be maintained. Flexamat may be mowed over. Flexamat can also be driven over with vehicles where access roads cross the letdowns.
Rip Rap is never a permanent solution where Flexamat is. Flexamat is also much faster to install compared to placing rock. Plus the ease of maintenace of Flexamat compared to rock.
Gabions are very labor intensive to install. None of the benefits of vegetation are possible. Many times, erosion will take place underneath the gabion mattress and they fail.
Poured concrete lined channels will crack with settling and freezing and thawing cycles. There is also difficulty to access the channels for concrete trucks. Additionally, stormwater increases it's velocity while moving through the concrete lined flumes. Any pollutants that end up in the concrete lined channel will be transported to the natural water way the stormwater is discharged into. A vegetated channel will cleanes these polutants before they are transported off site. 
Installing HDPE pipe increases the water's velocity similar to a concrete flume. There are none of the benefits of vegetation. The pipe is also difficult to manuver around for access roads while Flexamat may be driven on. HDPE is expensive and there aren't any advantages to using it.
Typically, Flexamat is manufactured at the landfill to reduce material freight. Many times, this makes Flexamat the best priced option, less expensive than rock rip rap, gabions, poured concrete, and other permanent erosion control options.
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