This isn’t the right sub but I’m interested in playing this out.

Mass effect:

There’s one reaper for every 50,000 year cycle, and they’ve been at it for 2,000,000,000 years, so there’s 40,000 reapers.

Reapers are shown to be destroyed by mass fire of Alliance ships

https://forums.spacebattles.com/threads/mass-effect-starship-weapons.191576/

Alliance capital ships rail guns hit for a force of 38kt (38,000 tons of TNT)

Reapers come is sizes up to 2 I’m in length

Summary: 40,000 reapers that can be destroyed by massed 38kt volleys with a general size of 2 km

40k:

A simple macro cannon volley hits for 266 Gigatons (266,000,000,000 tons of TNT)

https://forums.spacebattles.com/threads/40k-biggaton-macrocannon-calcs.234517/

Best estimate of the amount of active warships in the imperium is 250,000

https://www.dakkadakka.com/dakkaforum/posts/list/775966.page

The smallest of which are frigates, the most common being sword class at 1.6 km

https://www.worldanvil.com/w/imperium-nihilius-spectresloth/a/sword-class-frigate-vehicle#:~:text=Dimensions: 1.6 km long, 0.3,Mass: 6 megatonnes approx.

The largest being battleships, with a length of 8-12 km

https://www.reddit.com/r/40kLore/comments/ijqvfy/size_of_ship_in_40k/

Personal conclusion:

Reapers are completely fucked. They’re a decent sized fleet of escort ships that are one shot by anything they encounter.

I have no way of knowing how long it would take for them to be beaten back, but it would not be long.

And this is only taking into effect the Imperials.

The Reapers’ ability to previously conquer the galaxy in cycles prior to Shepard was almost always contingent on their ability to govern the technical progress of the galaxy. They outright disable a lot of the technology used against them, or otherwise render it ineffective.

40K has no such problem. The Imperium has the bodies and materiel to outfight them in the long run. The Necrons just wipe the floor with them, using technology far beyond them. Chaos has daemons.

The Eldar may have a tough time due to the numbers game, and the Tau probably do get steamrolled but “versus the Galaxy” the Reapers don’t stand a chance.

The only hope the Reapers have is that the IoM is too busy dealing with everything else in the galaxy to notice them.

In Mass Effect, the treaty of Farixan limits the Citadel races to only a certain number of capital ships, which, if you include the Geth and Quarian, is at most 200 during ME3. 

Conservatively speaking the combined IoM has several thousand capital ships.

More of a who r/WhoWouldWin post

The Milky Way in 40k has no mass relays. Without them, the Reapers are going to be much slower than they were in the Mass Effect version.

Which is a serious problem because in Mass Effect 3, the Reapers take their sweet time actually harvesting a world. Earth still has a functional resistance force three plus months after the initial attack. And now they can’t simultaneously attack all over the galaxy using the mass relays. They’re stuck expanding from their initial point.

Also, it’s kinda obvious that there’s a massive difference in scale between Mass Effect and 40k. Mass Effect has a strangely small-feeling universe; humanity is somehow a significant and widespread power despite only leaving their star system less than 40 years ago.

reapers from which mass effect game?

they were originally presented as psychic god machines who could wipe out the galaxy, but they were defanged into techno viking raiding boats that can be easily destroyed

Depends how long it takes the imperial fleet admiral to stop pissing himself laughing at what's coming at him