Credit goes to u/ooooNIXoooo for helping me out with the tests!

Ranged weapons

• Nocturno (Assault Rifle) [Meta weapon]

There is no doubt this is one of the great Assault Rifles. With the 6th perk, it's a great multi-purpose weapon.

No noticeable downsides of this weapon

• Founder's Drum roll (Assault Rifle)

A 50 round LMG that increases damage by 10% for each 10th bullet fired up to 5 stacks (50% extra damage on the 50th bullet), perfect for applying a mag size perk for that weapon

The downside of this weapons is having to add a mag size perk for it in order to have the 5th stack to work.

• Founder's Quickshot (SMG)

The older brother for thrasher. Another one of the top performing SMGs. Not to mention the 6th perk further increases the efficiency of this weapon

The downside of this SMG is you have to keep hitting enemies and keep reloading in order to keep the efficiency of this SMG.

• Founder's Deconstructor (Shotgun)

A slept on anti-tank shotgun that can further increase the damage on reloading.

The downsides of this weapon that you have to reload in order to increase the damage. And the next shot will use all the boosts.

• Founder's Revolt (Pistol)

The secret is within the 6th perk of this pistol, not only it's great against crowds. But the rate of fire is spammable with this pistol.

The downside of this pistol is being available at Epic rarity. means lower durability. But that doesn't stop it from being so powerful.

Melee weapons

• Blazing Masamune (Sword)

It's best utilized when adding 3 attack speed perks. Making it a great crowd control melee weapon if you are playing Paleo Luna.

The downside of this sword that it's locked to fire.

NOTE: Founder weapons are no longer obtainable since 30th of June 2020 due to Epic discontinuing founder packs

TL;DR: Some of these weapons are being left in the dust. But maybe because very few people obtained them all.