IEO is a new concept which unlike ICO. The difference is on the participation of IEO you will get your token credited instantly to your wallet which was not in the ICO. Also, you have to wait for traded the coin After ICO by waiting the listing of the coin in exchanges. But after IEO you will get to sell your coin after the finish of the token sale.
In as much as IEO has its full benefit than ICO, it has its own bad side effect too, it makes the project vulnerable to dumping immediately because the reason why the investors they got participated is to get profit from their investment and not because they so much desire to stay with the project.
If you look at it, people that will get involved more in IEO are mostly whales, who has the money to buy this projects coin in large quantity while they leave the rest to few innocent ones who would not get the chance to withdraw their money by the time the whales strikes by dumping immediately they get the desired profit level.