Interesting, I find that the "THXII Certified" audio system in my Lincoln is superior to the BOSE in the Envision. Not sure if your Flex limited has the same THXII system or not.
It may have something to do I suppose with sound deadening in the Lincoln, as referenced above. I will say that the Envision BOSE system is better than most prior GM BOSE systems I have had, including our current 2014 Impala. I am a GM guy and the Lincoln is the first and last Ford product I'll likely have. Love the Lincoln, but don't love the dealership service departments around me.
The last GM BOSE system I had that truly was incredible, was in my 2007 Escalade. I'm going to say this is the 2nd best BOSE system we have had (in the Envision). No complaints about it, but it just doesn't seem to have endless power like I feel I had the 2007 Escalade and have in the Lincoln. Admittedly, it's all subjective.......
2017 Envision Premium I AWD 2.0T (Summit White)
2014 Impala LTZ 3.6 V6 Limited (Summit White)
2010 Lincoln MKS AWD Twin Turbo; Livernois Motorsports tuned; 0-60 3.94 seconds (Steel Blue Metallic)