It’s always easy to ‘knock’ someone else’s idea, but assertive people are positive individuals who have a constructive mindset so, of course, will always be ready to provide a solution as well.
Rather than just saying, “I don’t think it will work”, they will say instead, “I am not convinced that, in the way that it has been proposed, it will work. However, I would like to put forward an idea that will really make a big difference to its success.”