He’s extremely powerful to say the least

For starters he’s defeated Horus who could create small Supernovas

Horus overpowered Sanguinius

And Later murked

And created the Lupercal court

And was just as infinite as the Warp(I will talk about the Warp later)

And had artificial infinity created by him, which as stated, is as infinite as the Warp

Now let’s Talk about The Warp

Which is outside of time and Space

A formless and limitless place

Which lies on the other side of dreams and is infinite in scope

That contains worlds which can be considered universe of their own

With infinite layers

Each Layer Transends one another in the Warp

And is a timeless metaphysical realm

He defeated a shard of Mag'ladroth the Void Dragon

Even though he couldn’t murk him

Due to C’Tan being a physical part of reality/the Materium

This is still impressive as The Void Dragon can create Star systems even as a mere shard

C’Tan are also 20D and can create 20D technology, and by extension the Void Dragon is 20D

So this is impressive anyway as the Emporor could defeat a 20D entity, even if he didn’t face the C’Tan at his full strength

He also created his own Imperial Webway

Which was an endless labyrinth

A formless place that’s infinite in size

Outside of space and time

So the Emporor of man could create that as well

And created the the forest after Lion returned

Which Lion can forest walk here anytime he likes and is theorised to be adjacent to the Warp itself

And he burned Nurgle garden through Guilliman

Speaking of Nurgle he is comparable to Nurgle and his buddies

All four of them

Can exist and manipulate higher dimensions

Can warp their own realm of chaos

Can create and recreate armies within the Warp

Can manipulate the Aethyr

can consume and absorb souls

Can bestow powers onto others:

Create Avatars in the Materium:

And can manipulate the Skein which is in-between the Immaterium and the Materium

Exists as a vortex like Khorne

And their existence cannot be defined

This includes Tzeentch

Tzeentch exists in the hidden Library which is an infinite dimension that contains all knowledge and constantly folds itself under it’s own weight

His mere laugh can mutate anything or anyone

He was considered the most powerful out of his siblings to the point that he was considered more powerful then his siblings until Khorne taught him a lesson

Which means at the time he scaled above Slaanesh who consumed the Aeldari pantheon(with some exceptions such as Isha who was saved by Nurgle)

Who have no defined existence

Exist beyond physical space and or causality and time

Can also affect the Warp

Can create Avatars which can force entities into fear and submission

Has plot Manipulation

can manipulate fate and causality

He can manipulate time and space especially within his own realm

With his realm being the most dangerous one within the Warp

And he is an always changing being with an indestructible form

This would also include Khorne

horne can nullify powers because he wants a fair fight with people

His visage can cause madness, fear or death

He can create portals and cut through dimensions using his sword

Can destroy daemons and souls with said sword

Throwned Skarbrand through time and space

Embodies Hatred, War, Destruction, Bloodshed, and Anger(as well as Martial Honour to add)

Murked Lesser warp entities

Him walking can shake the warp

Had already battled Slaanesh

Who has the potential, but yet to meet that, to outgrow the others sibling in power

And Khorne hurled Tzeentch into a mountain, when not only that his other siblings were trying to gang up on Tzeentch, but Tzeentch was considered the most powerful of his siblings at the time until Khorne intervened

And I think most importantly

Not only stated to be able to destroy existence

With his rage alone being able to destroy existence

And to top it all off his sword can cut down and end Daemons forever, Daemons can manipulate the Warp

Their are likely way more feats for this guy… but you kind of get the point…

This guy is overpowered as you can get, definitely one of the most powerful characters ever made, and unfortunately is downplayed a lot by people who doesn't know about him and his lore that much and is unaware of his true power.

Respect the Emporor of man

Edit:

He also keep the forces of Chaos at bay even in his current state

Which give a stronger case for him scaling to Nurgle and his buddies

Suleforger,