He doesn't have to load the card right away. He can sit on the rotation while CSing and you can't go near him without expecting him to lock in a yellow as you move to him and stun->Q you.
Q is amazing at waveclear. It is slow, but it's ability to clear the wave is strong. Also Nid's spear is slow and blocked by creeps. Slow doesn't always mean you're going to dodge it. Even pro's get hit by Nid spears.
Mana sustain is Mana sustain. His early game mana costs are not that heavy. And one of his abilities doesn't even cost mana to use, just a setup. Use it at the end of the a wave then back off so they can take advantage of it.
If you can stay warded 100% of the time and know where he always is, good job. But he doesn't have to ult from the river. He can move to near red/blue buffs. All he has to do is walk back to his tower then move where he wants to avoid you finding out.
He's weak vs every assassin because he's immobile. That doesn't mean he can't do something about it. He has stronger waveclear than most. He can lane safely while shuffling W. If the assassin roams he can follow very easily. As well as his ult can prevent ganks. Reason he doesn't get played in the LCS isn't because of assassins. It's because of the way junglers have changed. He can handle one opponent with his stun and farm safely. But two can make it too hard to deal with.