In my opinion the most important thing is to see who made the IEO, if it is a big exchange that has good credibility and reputation then I can say that they will not commit scam because if they do then their exchangers will be at stake and they may not want do that because even without scam, they can get a large income from their own exchangers even every day
If it's not in a trusted exchange, it will still look simply as a ICO then been listed in exchange fast. It's good to be listed in exchange already since some old project still unable to find exchange where their coins can be listed. But now that they start in exchanges what we need to think is how long that project can last in the market as well their team and goals in the future.
That's the point, the main selling aspect of why IEO seemed to be more legit than an ordinary ICO is the fact that it's listed by the popular exchanges meaning that they have been inspected by the exchanges, that includes document, developers identity and many more.
It's not a rocket science that more people will definitely put the trust into IEO rather than ICO, however whether the project will be succesful or not that's an entirely different matter.