For example, could I ask "is Duolingo/Rosetta Stone effective in teaching language X?"
We need to be careful about these sorts of questions.
Asking "Is X effective?" isn't a very meaningful question. To answer it affirmatively, you simply have to show that one person, somewhere, had success with the program. And how one defines "success" can be a contentious matter, too.
Generally, it would be better to ask for an academic study which examines the effectiveness. Such studies (at least when they're not financed by the company themselves) will typically talk about pros and cons of the system, usually comparing them to some other system (a control group).
Is Duolingo effective?
Not a good question.
Are there studies on the effectiveness of Duolingo?
What are the strengths and weaknesses of Duolingo's method?
Perhaps even better.