I haven't participated in a successful IEO.
But from what I observed, it seems to me that IEO's are attracting investors because obviously it is launched or have the backing up of a reputable exchanges. So for projects, no need to market it or at least spend a lot of money. As compare to ICO wherein there are so much hype around it and will open up bounties here in the community. But the sad part is that most of them failed and are not paying their hunters.
I seems, you need to do a lot more observations.
Note that not all IEO project token is listed or being sold on reputable exchanges, try to collect all IEO project and find out how many projects are listed on reputable exchange and how many projects are listed on shit and very unpopular exchange.
It seems IEO are helping investors to avoid scam project. It is maybe right but only to avoid scam projects not shitty ass scam exchanges.