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.Spike Growth is once per turn too unless you can move an enemy more than once on during your turn. Further it is an AOE and the enemy needs to be moved inside the AOE, not just moved anywhere, you take damage if you go inside it to move an enemy. This makes a lot of movement options less viable. If the enemy is standing at the far edge of spike growth, for example you have to circle around behind him to push him into it with repelling blast.
Ensnaring strike requires a bonus action and more importantly gets a save. Further that save is with advantage on a large creature. There no save on grasping arrow, it is automatic. This is a BIG difference.
Both Spike Growth and Ensnaring Strike also require concentration.
Finally Grasping Arrow is not a spell, so aside from the fact you do not need to concentrate, I could hit the same enemy with a second Grasping Arrow on the very same turn and essentially double everything and require 2 saves to break free.
I always thought people's main issue with the 5E Arcane Archer was simply for the fact that it's main "gimmick" that it has runs out too quickly and then your stuck with an Archer that hits/takes hits like a fighter and has Action Surge.
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.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.
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.
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.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.