You can think of ieo's as an upgrade of ico's, but both are the same thing in the end. Start ups that needs money for further development, and here we talk about the valid projects, not about scams. What is ieo actually? Well with ico is simple you check their official page and there you buy their coin/token directly from then, but now with ieo's exchanges are conducting the sales! In some way they vouch for the project, and if nothing else you know that after sale coin will be listed for sure.
But can we trust centralized exchanges? I don't think so, not all the time, not 100%, never! After all they take commission for everything they do, is it a sale, is it a trade, is it a listing, they make profit everything! Ieo's are just better on the first look, but after all its a crowdfunding, one way or another.