Talking for the difference, ICO usually find it difficult to get listed in an exchange or reputable one, especially when they are not sincere, not raising enough fund becomes their alibi. IEO has no issues with exchange at least.
You are very correct about these, ICO have challenges in listing because most of them don't have enough funds to pay exchanges for listing while IEO would be listed automatically by the exchanges at which they were carried out on, I think this is just the major difference between ICO and IEO, let us enjoy this IEO genuity why it last because I can assure you that in just few months to come, we would start seeing a lot of IEO scam projects.
But, as per my observation on crypto space in last 3 to 5 years, I am sure the success of IEO may last for another 2 or 3 years until finding out some other way of hosting initial cryto offering. But, success of IEO will be purely depend on how strict those exchanges on choosing projects.