A few points...
The FPSC system uses HELIUM as a working fluid, not CO2. Helium is a much smaller molecule than the refrigerant used in the Engel refrigerators, and is far more likely to leak out of the closed refrigeration system than the larger coolant molecules are. That's why mylar balloons that are helium filled deflate over time...the helium escapes.
Second, the Twinbird unit says
it draws 4 Amps at 12 VDC... 48 Watts. The M27 Engel unit (22 liters volume) draws a max of 2.3 Amps. The MT35 is a bit larger than the Coleman, 34 qt. vs 26 liter, and draws only 2.5 Amps. So, exactly how is the Coleman more efficient???
Even the MT60-combi unit (64 qt. interior volume) which has separate refrigerator and freezer sections has only a max draw of 3.6 amps, and gives you a lot more space.
Finally, IIRC, the Engel swing compressor system only has ONE moving part
... not three, so should be even more reliable than the Coleman system.