I think if after two years of study you still have no ability to actually communicate in Japanese, then it has actually been shockingly ineffective.

Ok, it's interesting because I also learned Japanese (MA graduate). In comparison, it took me a few days for learning hiragana/katakana plus few weeks more to read them a reasonable speed. Two years before that I've learned about 70 kanji with kunyomi in three weeks during high school. That part was done also with Wikipedia-sensei. Then I actually started learning the language at university.

So, if after two years of using Duolingo you cannot read all the katakana and cannot make a sentence more complex then xxx ha yyy desu it just totally prove how bad the application is. And what's even more damageable is that user think they are making progress and then do not focus on real, useful material.