Of course it would be ideal if an ICO project already has a working platform/product, so investors have something to test and not just the idea of a team, they have to trust. But often the projects depend on the money from the ICO to develop exactly this idea they have.
It is really hard to make a valuable product, when the team gathered right after the ICO. Cause a lot of forming processes emerge, not all people can work together and do it really efficiently. I think the only way is to invest into the project with the product and revenue. Cause in this cases, people prove that they can build something and there is a demand on the market of such projects.