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.
that only means bigger exchanges should be hosting bigger scams while small exchanges host smaller scams!
you have to realize that the exchange does not care at all whether an ICO is a scam or not. they only care about making the most amount of money. so they first charge a bigger fee for listing (a bigger bribe) then they also earn profit from trades that take place on their platform as everyone dumps (the trading fees).
so far in the past years that we have seen thousands of fundraisings in different names (ICO, IEO, STO,...) not even a single one of them have been legitimate. all of them are useless and got dumped after a while.