I agree. The out of combat movespeed isn't the issue here, and if anything it actually promotes these items' use on the champions that should be building them rather than the ones who shouldn't because assassins like to roam - that should be gameplay that's encouraged imo.
My personal opinion is that EoN needs to be changed so that it has a faster cast time but way shorter duration - maybe 1 second, 2 at most, and possibly a longer CD. I feel this would promote its use on assassins as their playstyle is already very dependent on quickly getting in, doing damage, and getting back out, while it would be less effective for ADCs and other champions. As it stands, it's highly effective on ADCs because they can pop it before the fight happens, and have a free spell shield that the enemy has to get through before actually being able to fight them for essentially the entire duration of the fight.
By the way, I think this is a problem on assassins too. You shouldn't just get to stand around for 10 full seconds waiting for the perfect time to go in with your shield up - once you pop it you should be forced to use it within the next few moments or lose it for the duration of the fight. The amount of standing around the item promotes is completely contrary to what assassins should be about.
As for Youmuu's and Duskblade, I don't actually think they're all that overloaded. The true damage on Duskblade is pretty insane, but it's also highly situational unless you're someone like Talon or Twitch, and Youmuu's is actually pretty straightforward. It's a stat stick that gives you some move speed on a reasonably long CD for what it does. You could argue that the vision denial part of Duskblade makes it a bit overloaded as well, but imo it's not that big of a deal. It's on a long CD and it's no substitute for a proper Oracle's Lens if that's what you need in the situation.