Here's a visualization of how you can think about arm strength for an NFL prospect.
The blue line is actually Drew Anderson but I'm not trying to tout him here, because in my experience Anderson just has a very typically ideal NFL starting caliber arm. He's not Patrick Mahomes, Aaron Rodgers, or Matt Stafford. But he's around the level just below that.
This is really just to show you visually that Will Grier doesn't stack up. And he's not alone. That people aren't necessarily singling him out in this year's class for having a popgun arm is very telling, because so many of the rest of the guys are in the same boat.
Slimm has it right with Gardner Minshew ranked above Will Grier. They have the same arm talent and Minshew is just a better performer on game days, with a much better character/football intelligence profile.
There are two aspects of the BLUE line that deserve mention, and describe what you typically see in the ideal NFL QB prospect:
1. At that 60 to 100 foot distance ranges, you need to see average velocities in the 50+ mph range. Anything below that is a big red flag.
2. Notice how the function is concaved. This is typical of a strong-armed player that is putting NFL caliber throws on tape at the college level. Guys with NFL caliber arms can maintain that 50 mph average velocity as you stretch out the distance longer and longer. But at some point, velocity breaks downward more steeply because the players have to really ARC the football high in order to get to that distance. The later that inflection point occurs, the more ideal the guy's arm strength is.
One difference between a Drew Anderson and a Carson Wentz, for example, is that I've seen Carson Wentz maintain 49 mph terrestrial speed even on a 140+ foot throw. I haven't seen that of Anderson. Doesn't mean it can't happen. It's just not on tape. That's what private workouts are for.