The Space Wolf novels of Bill Kings era are effectively retconned and ignored. The game and story has has advanced 12 years since then. Similar to how the rest of William Kings work has been forgotten and ignored in all but the spirit - Gotrek and Felix has a wibbly wobbly timey wimey moment - which soon turned around when it was just completely ignored by the continuation writers (although admittedly, I've not read much after the original run of books - after reading a review of Zombieslayer I kind of flipped shit), the Shadow-point and Execution Hour novels regarding the Gothic War contain information which is frequently countered in new fluff.
If you're willing to count such old fluff, you can count Sigmar as the lost Primarch, Chaos Space Marines in Warhammer Fantasy, Space Marines using Eldar weaponry, the Necrons were similar the galaxies version of a super effective Norton antivirus (everything a virus, but this time, it actually manages to defeat them), the C'Tan themselves could take the battlefield, Genestealer cults actually occur (these have been squatted - there's no mention of them IIRC in the latest nid dex' since perhaps 4th, if not 3rd edition)...
In regards to defining someone as plain - they can't be defined as "plain". Either they're lacking a description, instead relying on their job or role or actions to define themselves, in as much as their name and equipment (might as well call the guy with the Heavy Bolter "the Heavy" as near enough in every book, he's the same character with the same outlook and attitude, only a different name and appearance to make him different), same for "melee guy", and "sergeant", and "the cherry", and "the old timer", and "mr flash", or whatever other bank job tvtropes trawl you can come up with. Every once in a while we might get "Magnificent Bastard" (Sevatar), or someone else who makes a defining character, but as for appearances, they're pretty much limited to the differences in the armour, or from a human perspective - where all they can see is the armour, or how that all of the space marines share the same appearance - and their abnormal difference from humans.
Read Brothers of the Snake - it is one which makes a bit of a point about that, where the Astartes land on a planet and the human administrator says that they all look the same, but that Priad wonders how she can say that when one is tall and thin, one is stocky, etc, but short of opinionated point of view characters, we don't get that.
Mostly, when it's regarding something beautiful or disgusting, it's to make the reader know about the things they're killing - whether it's the lithe, beautiful daemonette, or the puking monstrosity of a plague marine. Very rarely is beautiful used in any term in 40K literature.
That a human would consider a Primarch to be beautiful says a lot - there's a lot of psychological research which has gone into love and attraction - and simply put in a pithy sound bite, power attracts - and there are few more powerful than a Primarch. For someone to spend so much time around them, they are going to be open to having more than professional thoughts - such as more about how they're acting thinking etc. You don't need to be the most beautiful looking person to fall in love, but the opinion of one who is in love is going to ignore percieved cultural "flaws" and see it as a thing of beauty - i.e Scars, or piercings, or tattoos, or beards, oddly coloured eyes, etc. If you fancy someone who is a 6/10, then when you fancy them, your opinion is that they're an 8.
What I find notable though is that a Primarch, who is to a Space Marine what an Astartes is to a woman can quite easily see a height difference, especially when booted and suited of maybe twice the height of a small human (perfect height, giggity). In the literature, most space marines are noted as being ugly, with lumpen, mishapen heads, overly broad and packed with slabs of muscle - maybe along the lines of something like Kai Greene
If you make him another 2 foot taller and twice as wide, with a fused ribcage, implanted subcutaneous (as I understand it) Black Carapace which (again, as I understand it) is capable of withstanding the impact of a bullet (similar to maybe today's lighter bullet proof vests in resistance, so not capable of preventing an assault rifle/armour piercing, but lighter rounds and preventing damage to internals from already slowed down shots. Bear in mind Kai Greene is in Bodybuilding for aesthetics, so will have built his body around the ideal of what is considered good lucking - so there will be mirroring and obviously a vast diet and supplement regime to keep him in shape to show off the striations and muscle valleys in the body.
A Primarch, on the other hand, is going to have been created in the image of the emperor, and vat grown - they are going to be far more human than an astartes is. A 10 ft tall human, yes, but a human nonetheless (look at Fulgrim on the cover of the Primarch's, for example). They are going to have an immense physique - with maybe the exception of the rotten body of Mortarion - but not necessarily one which is probably human - more akin to a supermodel crossed with an ultrafit special forces soldier, rather than a bodybuilder. Vulkan will have had the packed muscle of a blacksmith, of course, Angron that of a Gladiator (although because hollywood, is more likely to be ripped, than the fat gladiators of history).
These are not sourced, of course - just my own reasoned guesstimations of something that will not be written about because at the end of the day, most 30-40k novels are aimed at luddite levels of intelligence to appeal to broadest audience with bolter porn.