But it's 2 per SR until lv 15 and can only hit one arrow per turn max and if you use it again the first one disappears. You can't even use it on two different targets at once even if you take 2 turn to do it.
Yes. But that is the same number of spell slots a Warlock gets per SR. It is hald of the 1st level slots any caster gets per LR.
Also people are comparing it to spike growth, you get 2 arrows per SHORT rest at level 3. A Ranger can cast spike growth 2 times per LONG rest at 5th level.
Then on top of that unless the 10ft movement penalty prevents them from reaching someone it's just 2d6 damage which most NPCs will willingly eat over spending can action to attempt to pull them off. It's a delayed smite that can add some extra damage and if your party has forced movement you could get some use out it. Enough to matter maybe but most likely not.
No it is 2d6 per turn, not per round. So if you have a party of 6 and all have enemy movement options that is potentially 12d6 per round.
If you have a 5th level warlock with releppling blast for example can add 4d6 by moving it twice, on top of the EB damage. Thorn Whip, Lightning Lure, Crusher Feat swarmkeeper. All those do an extra 2d6 damage every time you hit it with one of them.
The reason why spike growth is better is it's damage is determined by distances so even something like thornwhip could add as much additional damage and the whole grasping shot does.
No. Thorn Whip will do 4d4 (10 points) extra when you cast it on an enemy and pulls him through 10' of growth. That assumes he is already in or standing on the edge and it assumes there is at least 10' of skpike growth between you and it. If there is 5 feet between him and the spiked area it is 2d4 (5) or if he is already in it and there is more than 5 but less then 10' it is again 2d4. Further you can do 4d4 a maximum of 2 turns in a row at which point he will be entirely through the spiked area unless someone else was moving him in another direction, this assumes he does not move at all himself.
Hit a enemy affected by grasping arrow with thorn whip and it does 2d6 (7). So it is three points less extra damage IF you can drag him through a full 10 feet of spikes. If you only can drag him through 9 feet of spikes it is actually less damage. You can continue to do this 2d6 hitting him with thorn whip every single turn, regardless of his positioning.
Moreover the enemy can misty step or teleport out of or around spike growth and once he moves out of it he can avoid it unless someone can put him back in it. Finally you can fly over it or dispell it. You can't do any of this with grasping arrow. If you get hit with it you are affected until you make your strength check or a minute passes. You can move by teleport with grasping arrow, but you are still under its effects after you teleport.
Finally it does not impede allies like spike growth does. You can damage enemies with spike growth, and they can damage you with it too.