Some thoughts here:
I've bought new Fords 1971. Stuck with simple, unsophisticated versions and never had issues. Always reliable and economical vehicles (mostly F100/F150, six with stick). People who bought the high-dollar, optioned-out versions of what I drove, often had serious recurrent problems. Conclusion: Ford does simple well. Ecoboost is not simple.
A small engine like the 2.3 will be into the boost quite a bit to achieve anything approaching normal road performance in a Mustang. This is obvious, no one would buy a normally aspirated 2.3 Mustang; the car is too heavy, it would be gutless. So, being into boost a lot, economy will suffer.
The 3.5 Ecoboost they put in the trucks would be a better choice. The 3.5 is enough to push the car around in a respectable manner without getting into boost, yet the power would be there when called upon. The 3.5 Ecoboost seems like a great match for the Mustang and I can't understand why it isn't offered...other than Ford enjoys raking in the big bucks on the 5.0s.
At high elevations, boost comes into its own. NA engines lose power with altitude, boosted engines don't. If I lived around Denver or some other high altitude location, I'd be sure to go Ecoboost as that would be the only way to get an engine that could deliver its rated power. Though, I'd probably go for a tricked-out 2WD, short-cab, short-bed Ecoboost F150 before I would a 2.3 EB Mustang.
Overall, I think the 2.3 Ecoboost will be gone from the Mustang line in a few years. Too little return in either performance or economy for the extra complexity of the EB engine. It will be replaced by a 3.5 or similar V6 EB that can deliver the kind of performance Mustangs are noted for without working hard.
First off, you could argue that it's easier for any car company to make a simpler car. But the problem is that there is no simple car anymore, and some of the added complexity is not by choice either. Increasing safety standards, consumer connectivity needs, and fuel economy standards are all demanding newer and more sophisticated technologies. Even subcompacts and econoboxes are now loaded with features.
Thinking that you will have to wring out the new Ecoboost motor to achieve good performance is a conclusion that doesn't really align with the facts we know so far. Car and Driver's primer on the three engines says that the Ecoboost will deliver 320 lb/ft of torque at 3000 RPM and will still be making 296 lb/ft at the peak 5500 RPM. By contrast the old 4.6 Modular makes 320 lb/ft at 4500 RPM.
So, we have a flat torque curve +
-More balanced chassis
-Much better weight distribution (52/48)
It will not be hard to make this car go fast.
Turbos have come a long way. Serious lag isn't an issue anymore, and tuning and design can produce great turbo power at high RPMs AND flat torque curves for usable low end power.
It won't be replaced any time soon. This is the future of performance cars with the new CAFE standards. Lots of forced induction 4s and some V6s. If anything, the V8 will be the motor to go in a few years, replaced by a high output V6. The CAFE requirements are going to increase year after year. No getting around it.