A questionnaire can be a very helpful way of getting real peoples opinions on your ideas.
However one of the disadvantages is that although you are putting in options you have chosen, you could have a particular vision for your magazine which might not be voted the favourite option, therefore meaning you would have to compromise to go with the most popular vote.
Also, another disadvantage with a questionnaire is that the people who fill it in, may not be filling it in with valid information, for example, they could fill in an answer for a joke, which would mess up the final result of the questionnaire.
With all these disadvantages, it is still a very good way of researching peoples opinions, without you having to guess what they might like. Most of the time, it is accurate, and reliable information that you can use to adapt and use to make your magazine interesting for the target audience.