The damage boost makes the weapon do more damage than delta. Nornfang does way more damage to vehicles than delta does. No doubt, it is the best sniper available.
30 meters, aka 90 feet, is long range for what's supposed to be a close quarters weapon. No doubt, it's the best hammer available.
Whispered truth has the fastest killtime of any sidearm. No doubt, it's the best sidearm available.
SPNKR Prime has insane projectile speed, more ammo, and makes the user incredibly fast and also tankier. No doubt, it's the best rocket launcher available.
Jorge's chaingun has a ton of ammo, and makes the user incredibly fast and it shreds both vehicles and spartans. No doubt, it's the best turret available.
Prophet's Bane has increased lunge range, and movement speed, and makes you more stealthy. No doubt, it's the best energy sword available.
Do I need to go on? Is it really not obvious to you that the mythic weapons were within reason, supposed to be the best available?
The Nornfang does the same amount of damage even with the Damage Boost as without it it is simply a Sniper shooting Hydra missiles thus the Damage Boost is required to be a one-shot kill weapon.
Correction: The Gavel's Shockwave has a range of 21m. Also, the Corpsemaker or Grinder are better due to the Corpsemaker having the fastest swing time of .5 seconds and longest un-/scoped lunge range at 9.7m/13.5m, or the Grinder due to it's Gravimetric Vortex and EMP effect on Vehicles.
The Whispered Truth's Fastest TTK is only when it is in 35m, due to it's wide bullet spread, otherwise it has the longest TTK of any Magnum, so the CE Magnum is the best Magnum for a consistent TTK of .75 seconds at all ranges and it has a Speed Boost equal to the Truth's, has no recoil unlike the Truth, and you can get more kills out of it per magazine as it takes 3 headshots (bullets) to kill unlike the Truth's 2 bursts to the head (6 bullets).
SPNKr Prime does have a faster projectile velocity and 2 more rockets than other SPNKr variants but the EX is better due to it's Proximity Cluster Bombs and can do damage to multiple vehicles at once as well as having the largest kill/blast radius of any Rocket Launcher, and the EM is better due it's 4 rocket cartridge and ability to lock on to ground vehicles. Also, the Ad Victorian is better due it's guided rockets allowing it to hit enemies behind cover and having the same kill/blast radius as the EX as well as the Proximity explosion, and the High Five has the longest RRR via x4 Scope at 350m as well as being able to lock onto Ground Vehicles and Infantry.
Jorge's Chaingun has the same ammo capacity as any other Chaingun but it has a shorter cooldown and longer firing time. Also, it only makes you as fast as a walking Spartan but you sacrifice the advanced mobility which is essential to map traversal in Halo 5, but the SAW variant "The Answer" has the exact same TTK, doesn't sacrifice the advanced mobility, has a longer RRR, and can kill more Spartans overall due to its damage radius and 15 kills per drum.
The Prophet's Bane and Vorpal Talon have the same un-/scoped lunge range at 9m/15m, and all Swords give you a Speed Boost with the Bane being slightly faster at walking which is useless since the Camo isn't engaged properly unless you are crouching unlike a standard Camo power-up which allows you to walk with Camo fully engaged. Also, the Vorpal Talon is better due it giving the player 2 thrusts per cooldown unlike the standard 1 and it doubles your Jump Height and Stabilization time unlike the other Swords, and the Ravening Sliver is better due to it's faster swing rate of 2 per second allowing it to win all Sword fights and it has a faster draw time.
So, Mythic weapons are not the best weapons in the game as they are easily outclassed by other weapons such as the Answer and Ad Victorian.