I hear this alot and I am sure so many people underestimate the size of the cone, so, I done some testing.
https://imgur.com/oGo41b2
This shows the size of the cone using striking dummies. I was in the corner of the wall in the bottom right, I was aiming at the dummy at the top, which is at max range fyi. The left most dummy shows max range in that direction, the one slight in is just there for something to hit (like line AoEs, cones suffer from the fact that, if you hit at max range on a dummy, it will not register, this is an issue only on dummies though).
So, attacking the top dummy, do you expect the dummy in the gateway to get hit? Of course you don't, however, it does. Pretty sure that is alot wider than you thought right?
Going onto the next image:
https://imgur.com/p4v6eoQ
The top and left most dummies are in the same spot as the top and leftmost ones in the cone picture, that third dummy however shows you the range on a player centric AoE of 5y. Quite a big difference.
This last picture is just a reference, and shows how much of a difference the discrepancy is between the max range cone and the max range it registers a hit.
https://imgur.com/iXB6qr0
All the pics were taken from top down at max zoom, so in theory they are all the same scale, I would add an AoE to better indicate the range, however, I'm not that good at image manipulation, even if it is just slapping a shape onto it.
(For some reason it didn't imbed the images, so, unfortunately, you are going to have to click the links).
Bonus:
Because I'm pretty sure people will say the first image is fake, I pulled out my super secret video skills.
https://www.youtube.com/watch?v=fRdI30BcrSo
As always the dummies furthest away show the max cone range, there is a dummy in the gateway (probably not in the same place as the pic) to show that that is indeed in the path. Video starts with me tucking myself away in the corner, which I how I made sure I was always in the same spot. I showed the far dummies are in range of Grim Swathe (the soul cost is white, not red), I Harpe so that I know I am facing the dummy at the back dead on and, without moving, use Grim Swathe, which yes, does have the same range as old Overpower. The dummy in the gateway gets hit, clear as day. That cone is definitely bigger than you thought it was going to be.