Ofcom insists OnlyFans, Snapchat and Twitch have "impenetrable" site rules.
The watchdog has assessed how easy it is for people to access, read and understand the terms and conditions set by six platforms - also including TikTok, video sharing platform Brand New Tube and BitChute.
Jessica Zucker - Ofcom's online safety policy director - said: "Terms and conditions are fundamental to protecting people, including children, from harm when using social video sites and apps.
"That's because the reporting of potentially harmful videos - and effective moderation of that content - can only work if there are clear and unambiguous rules underpinning the process."
The report found that the length and complexity of some meant they weren't suitable for children, as they require advance reading skills to understand them.
She added: "Our report found that lengthy, impenetrable and, in some cases, inconsistent terms drawn up by some UK video-sharing platforms risk leaving users and moderators in the dark."
OnlyFans has the longest terms of service at almost 16,000 words, which would take over an hour to read.
Twitch, Snapchat, TikTok Brand New Tube and BitChute follow behind, with the latter's 2,017 word taking eight minutes for the average adult.
All of the platforms - besides TikTok - were calculated to be "difficult to read and best understood by high-school graduates".
However, Ofcom still noted that while users without university education may understand TikTok's rules, they could be challenging for the platform's youngest users.
In response to Ofcom, a Snapchat spokesperson told the BBC: "We share Ofcom's goal in ensuring our community guidelines and terms of service are easy to understand for everyone who uses Snapchat.
"As Ofcom recognises, we have a number of good-practice measures in place, including using reading-ease tools to regularly review language.
"We are in the process of updating our guidelines, including adding more information about moderation and what content is and isn't allowed. We will continue to gather feedback and work with Ofcom to ensure our rules are easy to understand."
