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.
I don't think we should generalise this. Of course that the vast majority of ICOs and IEOs have been proved to be scams, but there were some projects that made a good job with the raised funds. Look at Ethereum, it wouldn't be here without the initial funding. Another case is Stratis: they have raised what.. $750k? And they have built premium blockchain products from the ground up. That's what I name a successful ICO. When a project usually has a crowdfunding target of tens of millions it is a red flag for me. Usually this kind of financing is done through private investment, not crowdfunding.