So, I just watched the "How not to fix things" video. While the host is really clear and the background has really weird artifacts. Now, my screen is always not very good with dark areas (ASUS ROG Strix G713QR Laptop Screen) but this was never as pronounced as in the GameLinked Background. Trying to screenshot it shows the issue a little bit but in reality it is way more present:
Youtube settings:
This wavy pattern happens everywhere in the replaced green screen background (In dark but also in light areas) but never in the foreground (Even in the dark foreground areas it is not present). So while the green screen cutout is pretty much perfect without any green ghosting or "wobbly" edges these artifacts break the perfect illusion.
My conclusion: I love GameLinked and it's content. If anyone would ask me how to improve the production quality of the GameLinked videos I would answer "fix these artifacts".