the only difference is the middle letter. ICOs are scamming people by calling themselves initial "coin" offering and IEOs are scamming people by doing the same exact thing while calling themselves initial "exchange" offering.
the end result is exactly the same with different speeds at which they dump and exit. as IEOs are already supported by and listed on an exchange they can dump and run away faster.
it means that scammers will more easily deceive investors by slisting their garbage project into exchange, because IEO is more trusted than ICO.
But for large exchanges the likelihood will be very small, because scammers will not want to pay large fees only for listings in large markets. Scammers only do IEO on cheap exchanges and do not require conditions to bother them. But the most important thing is research first.
We can more trust in IEO because, it will be directly be listed in an exchange, in ICO, it took them months or year before it happens, some of them does not list their coins and run away with their money that has been raised. In IEO we may see the exchange that they will be listed right away.