These examples are exaggerated.
Nidalee got a nerf to her traps because % health clears camps to quickly. In case you have not yet noticed, most champions with % health damage such as Gragas has a cap on it against monsters.
That is not to mention that Nidalee spears are prone to turn invisible half way through.
As well, the nerfs to Sejuani would only affect those who are new to her and can't reliably land all her skills. The damage shift might actually be a buff to her early game. Her ult in its current form pretty much guarantees an ACE if she lands it on more than 2 people.
Both Nidalee and Sejuani enjoy high win rates, while Lee Sin sits at around 46%. I know that it isn't all that matters; however, from a statistical standpoint, Sejuani and Nidalee are much better than Lee Sin and are allowing NEW players to win despite having less skill and practice.
On a side note, Vi's early game ult damage was recently nerfed.