you anwser it right , the differnece is how they market the project . IEO mostly success because they become partner of that exchange where they offer there token.
IEO mostly succeed because of the exchange site so basically that depends on the site wherein you launch the IEO. So therefore, not because you do an IEO then it will be a success.
You should choose a good site however it will really cost you a fortune. While some offers free listing to their site but don't expect a sold out.
Success for IEO today depends on which exchange they conduct there IEO since if they list on top exchange for sure the hype will start and people will buy and support and we can see as the example on Binance Launchpaud where many Owners earned huge success.
And also choosing a good exchange really an effective tool since if they choose to select the low level one for sure they will failed and there project will be a total flop.