I think other manufacturers also produce lenses that are decentered to varying degrees but people don't check or notice. I'm not saying that issues don't exist with Samyang lenses but I don't think its unique to Samyang....
Unless I saw evidence I'm not sure i believe they hand out any more free lenses to reviewers than other manufacturers do either - of course I may be proved wrong here.
Other than Samyang I have only ever had one other decentered lens which was an old used lens that had probably picked up a knock somewhere along the line. I have never had a lens purchased new from any of the other manufacturers that had issues with decentering. I test all my kit thoroughly because I need it for work so it has to be perfect. In the last 10 years I have probably bought around 100 lenses or so probably more. Maybe I have just been lucky.
I know that Samyang give out lots of freebie gear because they offered it to me based supposedly on my "influencer status on social media", that is what they said any way. I am in no way shape or form a social media influencer. All I had to do to get free gear was to promote their products. They specifically wanted interraction on Instagram. I don't even have that many followers about 13k or so. To be fair I did consider it but they also wanted YouTube video reviews and I wasn't down for that and it seemed like it could potentially take up a lot of time. Plus they actually wanted us to use the gear for work and I wouldn't be happy to rely on it.
A guy I know very well used to be a Fuji X photographer, everytime Fuji brought a new model he would be one of the guys who would test beta cameras, write reviews etc. He no longer is, not sure why, I think he just got fed up with how much time it took. Fuji never gave him anything, not one thing. Every single piece of equipment he tested had to be sent back. I also know a guy who is a brand ambassador for Nikon, I don't know him well just Facebook friends etc. he also gets pre production cameras to test and they lend him equipment, everytime something new comes out they send him one, but it all has to go back.
Samyang offered to send me whatever I was happy to promote for free and you get to keep it too, I don't know of any manufacturer that does that. Thats why I always laugh when I see YouTube reviews for Samyang equipment and they say the lens was sent by Samyang just for testing. Yes, you have to send back pre production stuff as it gets shifted to several different people, but once it goes into production they send another one for testing again and it is given to you free of charge. I know lots of other photographers that Samyang got in touch with offering the same thing, it was actually a running joke.
I only know one guy slightly who took Samyang up on their offer, funnily enough he also represents Tamron, a camera bag company and Benro. Samyang is the only one that gives him free stuff to keep, well other than crappy merchandise that all the manufacturers give.