The early 3.5 Ecoboosts had a lot of problems and were particularly susceptible to sludging the intake valves IIRC. They've improved quite a bit, the 2.7 Eco is a pretty darn good motor from a power and economy standpoint, probably what GM was targeting (but missed) with their new 2.7 I4. As mentioned the revised LT-based 5.3 gets almost the exact same mileage with more power and better prospects for longevity.
I had a Regal GS with the 270hp 2.0T, with a GM supported performance tune would do about 300hp, reasonably quick but would knock down 32-34 on the highway, which is 10-12mpg better than my ~450hp LS3 SS will do at a similar curb weight. T4's have their place and can be fun.
2016 Traverse (KIA @ 25k miles)
2010 Traverse (traded at 117k miles)
2016 SS Sedan
2001 Trans Am