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.
The difference between ICO and IEO is that in ICO, tokens are sold directly from the project website, and with IEO via exchange. The difference between IEO on reputable exchanges and on shitexchanges is that IEO on reputable exchange gives the impression that the project is more credible.
None of these options guarantee the intentions of the creators or that the project will succeed.
For me, the future of collecting funds for project development is STO, because a real company must be behind the security token and the investor is protected by law. And after such an invasion of scams for investors will be the most important security, which at this time only STO gives.