Would people pitch a fit if I brought in a HH ahriman to play in my traitor legion using regular ahrimans datasheet?
As long as it's on the correct size base for 40k and you do something like put him on a rock or some kind of scenery to make up for the extra height it's missing(his 40k model is on a flying disc, the on foot version got discontinued), no one will care.
I find it a bit silly that HH has all these cool primarch models and such that can't be used in 40k.
Because most of them don't have 40k rules, and most of the ones that do are now huge fully corrupted daemon primarchs(magnus, mortarion, angron, fulgrim). Guilliman's 30k model kinda sucks(it's mostly terrain) and the Lion's loadout isn't correct(40k lLion has the emperor's tilt shield, 30k doesn't). The rest are all either dead(Ferrus Manus, Curze, Horus, Sanguinius, and either alpharius or omegon but that's a whole mess by itself), missing(the khan, vulkan, russ, dorn), or busy jerking off(lorgar mostly hiding from corvus corax who is now a demon harassing lorgar... again it's a whole thing, perturabo, the other twin either alpharius or omegon that isn't dead).
Guilliman and the Lion were the only 2 where people knew exactly where they were at(guilliman was in stasis on ultramar, the lion was literally just asleep in the DA flying fortress monastery like sleeping beauty and no I'm not kidding it's that silly). Perturabo might be back due to Vashtorr, Lorgar might get off his ass which would prompt Corax to leave the warp to keep chasing him, The Khan might get found in the webway, Russ went into the warp and is supposed to be back for ragnarok(aka "the wolf time" because gay furries), and Vulkan's sons are on a quest to find his shit to re-summon him back into existence like exodia because he's a perpetual(and just eventually respawns after getting killed, because last known he was killed blowing up a power plant fighting an ork that was almost a krork... krorks are their own thing and the war of the beast is a stupid storyline)
Why is this? Is it purely because of scale of the models or something else?
Something else. As long as GW has been around, they're run in an incredibly stupid way with separate departments that are almost treated like they're competing with eachother at times. Additionally, selling 30k models means people are playing 30k but that assumption doesn't work if people are buying 30k models to play 40k with. Yes, that's fucking retarded. Initially the daemon models being mostly the same between AoS and 40k was seen as a benefit(and still is by consumers) but because GW is dumb it's basically a "liability" at this point and they treat them like a weird red-headed stepchild in 40k without a codex but still have a half dozen detachments so they may as well have a codex(some armies with a codex only get 4 detachments). This is actually an ongoing problem with a bunch of space marine models(that were made for HH but given 40k rules) sent to legends(so never updated, will eventually get written out of legends, etc.) and people wondering what's going to happen with half of the imperial/chaos knight models that were still forgeworld(as in, resin and not plastic, I'll get toe FW in a bit) that got remade into plastic but boxed as 30k models, and half of the custodes range that's also FW resin(and GW has said will be getting updated somehow with their 30k book coming late, but that could also mean their 40k rules going to legends but even for 30k they were missing half the models they had rules for, it's weird).
The reason GW operates like this, as stupid as it is, can likely be traced back to their beginnings selling D&D shit as a game store, then Citadel miniatures and paints became a thing as a business unit. When they became GW the company they're known as today, they still had licensing deals with overseas manufacturers(as in the US) making models for shit that GW didn't produce models for. This is Forgeworld/Armorcast and basically they were making titans and vehicles and such(there were a couple of companies doing this with a license). Eventually the originaly Forgeworld disappeared, and then GW spun up their own Forgeworld division to make lower volume resin models(characters, titans, etc.) and eventually even Horus Heresy game minis(before HH2.0). It was treat as a separate business(even though it wasn't) where rules were in separate books(imperial armour books), separate website, separate employees, and so on. Eventually GW dropped the forgeworld name(so that's what the "15+" tags are for on the warhammer site, they're the previously FW models, and they're resin which is separate from finecast which is a shitty resin GW did to replace their metal minis but it was awful and dragged on for years... again its own thing) and merged the sites together into warhammer.com but still treats the FW stuff like some weird bastard son in a basement.
You might be thinking that it's crazy GW has survived this long operating like a fucking mess of a company, and you'd be right. But the only reason they were able to get as big as they are is because their competition has generally been worse about running their own businesses(there's a bunch of discussion about this in the tabletop community thread)
