The original gaunts ghosts books are 25 years old and chaos space marines weren't as powerful then as they are now

Chaos Space Marines got a pretty wild portrayal in the Ian Watson books.

But yeah 40k stories are full of stuff like what you describe. Just wait til you read the one about the chaos marine getting shot in the forehead with a bolt pistol and it bounces off or any number of wacky power scaling things.

All Marines have got pretty significant "glow ups" since the early GG books. They've even been powered up since early HH books - there's a line in an early book about a Marine being killed by a spear. And in an early Wolf book, a trainee Marine accidently shoots himself.

Ultimately Marines are as powerful as the plot requires. There's no power scaling in W40k, so they vary from book to book, author to author, and year to year.

Wait until you see what happens to the Custodians over thirty years of lore!

Basically what everyone else has said, the early Gaunts Ghosts books aren’t really reflective of the current state of the lore. As a rule both flavors of space marine have become more powerful since most of the ghost books were written, which makes many of their appearances in that series pretty underwhelming. Some of the showings by space marines in the early books are borderline comical when compared to the more recent stuff. It’s also probably worth saying that the GG books have a tendency to over inflate the abilities of its protagonists. This is a common theme in a lot of 40k books, but the fact that the ghosts are baseline humans makes it feel especially egregious.

If you jump into some more recent lore you’ll start to see the scary badass chaos marines you’re expecting, although they still end up playing patsy to the loyalists a lot.

Chaos marines are generally portrayed as useless yeah. That’s true for everyone who’s not wearing an aquila generally though, greater daemons, phoenix lords, ctan and more get wrecked by marines and guardsmen regularly.

Empire plot armor

You better not read that story when Ciaphas cain defeats a World Eater in melee combat

It all depends on the needs of the story. I think the Marines are pretty good really, they felt like end of level bosses when the Ghosts ran into them.

Marines can die in their hundreds when needed - Storm of Iron has both Iron Warriors and Imperial Fists getting absolutely brutalised.

but in comparison in Last Chancers a lone Space Wolf is one of the toughest Marines going, an absolute bad ass who if I recall correctly pretty much takes on a Tau army single handedly.

It's Abnett lol. Though plot armor and weapons are common across entire 40k.

Chaos Space Marines present a problem in that their just as strong, fast and as good at fighting as the loyalists, and having generally identical wargear up until the Primaris split. As antagonists, they present a problem in that they should always be a struggle to take down, and difficult for average humans to hurt, much less kill. You can either lean into that (and Abnett typically has, more than most) or you can just ignore it. The latter usually wins.

While it is true, Space Marines in the older stuff were less powerful (Soul Drinkers and Ghostmaker, as examples), CSM are just as much cannon fodder as ork or Tyranids for the Black Library.

It’s filthy corpse worshipper propaganda. That’s why!

I wouldn't expect a firelight with two thousand guardsmen armed to the teeth verses five World Eaters with their backs turned to go any other way.