I think you should tell them, but not in those words. Carefully explain your objections. At worst it will be a good filter for teams that don't have the necessary commitment to carry through on their idea. If they are smart, they'll consider what you say, see what applies, and make any modifications that seem justified. If they quit, it's better they give up early in the process, because there will much more discouraging things hitting them down the line, and if they can't handle a few reasonable objections, they aren't going to make it even with a good idea.