[Edi]

The scout not being able to hide under siege is WAD.

The black heart not so much so, but it's an unintended consequence. Normally non-stealth units cannot be in a province under enemy control. A siege is an exception, since the province is contested. And assassination requires being present in a province controlled by the enemy.

So in a way it is WAD because I don't see any way to close that loophole unless units with assassination are required to have stealth the same way you can't mod units with holy magic unless they also have the sacred tag.