I do think that for many languages there are more effective ways to learn in a given time period than Duolingo, but if Duolingo helps you stay motivated and keeps you practicing in a way that other methods couldn't, that in itself has a ton of value.
I totally agree with your point, though, that some languages are better suited for Duolingo style of learning than others. For example with Russian it was quite difficult without any grammar basics explained so I needed to additionally read about it, while with Danish it was much easier to grasp the mechanics. Not quite so with the pronunciation...
For speaking, Duolingo speech recognition isn't very good and is not going to provide specific feedback for improving your pronunciation. I'm not even a fan of practicing pronunciation virtually with a live tutor. I have found in person pronunciation practice to be very valuable compared to doing the same thing but online. A textbook doesn't solve this and neither does Duolingo.
I don't know anyone who did stick with textbook approach long enough to learn something.