The question is not whether the majority of designs available have copyright issues, but what portion of sales are based on designs that infringe third party copyrights, trademarks or personality rights. Unfortunately, it's impossible to know the size of the company's potential copyright liability just from looking at the site
Simple searches on the site for popular intellectual property, such as the game Fortnite (https://www.redbubble.com/shop/fortnite), make it clear the company has liability risk.
