To get back on track, here are 4 thoughts on ECO:
1) ECO mode changes how the transmission shifts, but what else? I think it changes the responsiveness of the gas pedal too.
2) It's pointless to evaluate ECO performance using highway driving, where the car doesn't shift much at all (transmission sits in 6th gear the whole time) and you don't reposition the gas pedal. Steady highway cruising should use the same gas whether ECO is on or off.
3) You need to resist "fighting" how the car feels on ECO with the lower shift points and slower gas pedal response. I think some people don't like how it feels (you are saving gas so the car will be less peppy) and "overcompensate" by pushing the gas pedal even harder (like when they want to make the car downshift to feel peppy, they burn more gas by pushing down the pedal more, to force the downshift right then even though ECO is saying no you should be getting into and staying in higher gears ASAP).
4) You can use your RPMs as a guide to save gas and help ECO do its thing. Just ease off the gas pedal when you reach a target RPM, like 2500 for example, and let the transmission upshift early and resist poking the gas pedal to force downshifts.
I'm interested in hearing more if more recent versions of the car use other techniques when in ECO mode, but my knowledge is based on my 2012 year. I try to adopt a money-saving mindset when using ECO most of the time, and I turn off ECO when I want to enjoy the drive.
Another consideration is whether your transmission has learned very aggressive shift points based on your driving history. Maybe consider giving ECO mode a new try, and reset your shift points. Then, as you drive more conservatively with ECO mode on, you'll enable the transmission to learn new shift points that saves even more gas.