Which makes it ever more hilarious when they get killed by normal humans.
Black Library authors have been trapped between needing humans to be relatable and not having the saga cut short by a single realistic space marine.
That's one of my problems with 40K fandom. They would cherry-pick bits from the lore to make the Space Marines into these demigods of war, and some bits of the lore portray the Space Marines as demigods, but when you look at the lore as a whole, there's instances of marines getting decapitated by lasguns, there's marines who get killed by regular folks, some author who wants their special OC to be remembered has said character kill a Space Marine even though they're just a normie, and when you see Space Marines fight in GW's visual media, they fight well, but not that much better than a Jedi or a Sith.
Shit, the fucking Prequel Jedi make them look like paraplegic turtles, and those movies were shot with real actors doing those stunts! Somehow, real actors doing stunts captures the essence of augmented humans fighting better than CGI trailers and animations where you can make the characters do anything.
It kind of kills the hype when the fans over-hype the Space Marines too much, only for the actual thing that you can see to under-deliver. At best, they fight like Warcraft Paladins with guns, especially since Warcraft and Warhammer Fantasy have a lot in common, and 40K is just Warhammer Fantasy's sci-fi counterpart. But they're not the invincible gods of war that the fans hype them up as, especially when you actually see them fight.
At least when the odd Rebel or Clone Trooper kills a Dark Trooper or a Super Battle Droid, those were just souped-up killer robots. The space equivalent of the T-800 from the Terminator. They're not demigods of war; not even the Mandalorians or the Jedi have that status. If some Clone Trooper or Rebel agent kills a Dark Trooper or a Super Battle Droid in battle, they just killed a powerful enemy, but an enemy that's not invincible.
No Mandalorian or Force-user is invincible either; even before Order 66, many Jedi died fighting the CIS Battle Droids. Part of the reason Palpatine triggered Order 66 in the first place was because he feared that one of the Jedi might be strong enough to kill him, and wouldn't you know it, Mace Windu was one amongst those Jedi, so it's best to kill them all in a single surprise attack using their own troops, rather than face each Jedi man-to-man in a duel. Let Vader handle that shit.
In the Old Republic Era, the great Sith Emperor Darth Vitiate was struck down by some Jedi rando who graduated top of his class in the Tython Jedi Academy. And in the post-ROTJ era, Marka Ragnos, the most powerful Sith of the Old Republic Era, was defeated by Jaden Korr, some random Jedi minion of Luke Skywalker, who was training under Kyle Katarn. It goes to show that even the most powerful of the Sith can fall victim to some random Jedi.
And in the same way, no matter how much beskar the Mandalorians cover themselves with, they can still get killed by Republic troopers, random aliens, or even Stormtroopers. The Mandalorians lost on Dxun and Malachor 5 against Republic troops who were weaker than them, less heavily-armored than them, led by Jedi who were just pressing them forward like commissars forcing the Imperial Guard to charge forward. Some random natives that worked with the Death Watch kicked the shit out of Jaster Mereel's Mandalorian forces. The last major battle the Mandalorians partook in was the Battle of Botajef, where they lost hard against the Stormtroopers of the Galactic Empire.
The last battle the Mandalorians fought in the entire Star Wars timeline wasn't some glorious last stand against a superior foe; the Empire didn't send armies of Dark Troopers or Sith Warriors to take on the Mandos; fucking recruits in plastoid shells took them down, despite their beskar'gam and all their skills and training. It goes to show that no amount of power will allow you to dominate the battlefield when the other side has superior logistics and numbers against you.
Or as Mr. House from Fallout New Vegas put it, when he described the power-armored paladins of the Brotherhood of Steel: "Ideological purity and shiny power armor don't count for much when you're outnumbered 15:1."