I wonder what the worst single Idea in the game could be.
Cavalry flanking has to be up there.
Increased coring cost against is probably my choice. Even if you lose a war, does it benefit the player at all? It is just an annoyance to the player and I am glad the removed it from most of the world (I think just a few natives have it now).
Throwup between
+20% Female Advisor Chance (Mantua)
or
-10% Cost to Justify Trade Conflict (Genoa/Venice)
I'm pretty sure the 1.3.1 patch will also ship with the ability to give custom nations Transport ship combat ability, which probably takes the cake for worst combat related idea.
Convert-only ideas are, if the wiki is right, either mediocre or amazing. On one hand you have the Templars with 10% discipline and on the other the Aztecs with 5% tax. https://eu4.paradoxwikis.com/National_ideas
In terms of vanilla, I'd rank embargo efficiency very high on the list of useless ideas. That and -5% shock/fire damage taken (if you compare it to other army related modifiers).
And while I'm at it, can I call your attention to how bad Hussite is as a secondary Tengri religion? It gives you an amazing -2,5% shock damage taken. That's it.