Many users of video-sharing sites such as Onlyfans, Twitch and Snapchat would have difficulty reading and understanding the site rules, according to the media regulator.
Ofcom has looked at how easy it is for people to access the terms and conditions set by six platforms.
It found advanced reading skills were needed to understand them.
It also found their complexity and length meant they were unsuitable for children.
Jessica Zucker, online safety policy director at Ofcom, 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," 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."
According to the watchdog, at nearly 16,000 words, OnlyFans has the longest terms of service, which would take its adult users more than an hour to read.
This was followed by Twitch, Snapchat, TikTok, Brand New Tube and lastly BitChute, which – with 2,017 words – would take eight minutes for the average adult to read.
Ofcom calculated a "reading ease" score for each platform's terms of service. All, except for TikTok's, were assessed as being "difficult to read and best understood by high-school graduates".
Although TikTok's rules were likely to be understood by users without a university education, Ofcom found the rules would still be challenging for the youngest users on the platform to comprehend.
The report also showed that Snapchat, TikTok and BitChute use "click wrap" agreements – where platforms make acceptance of the terms of service implicit in the act of signing up.
Because users are not prompted or encouraged to access the rules, it makes it easier to agree to them without actually opening or reading them.
Ofcom's study also identified that users may not fully understand what content is and is not allowed on their sites. It found OnlyFans and Snapchat provide little detail to users about prohibited content.
The study also found that users were not clear about the consequences of breaking the rules set out by the platforms.
While TikTok and Twitch have dedicated pages providing detailed information on the penalties they impose for breaking their rules, other providers offer users hardly any information on the actions moderators may take.
The Ofcom report looked into content moderators on the six platforms and learned that they do not always have sufficient training in how to enforce their terms and conditions.
The quality of internal resources for moderators varied significantly between the sites, and few provided specific guidance on what to do in a crisis situation.
"We share Ofcom's goal in ensuring our community guidelines and terms of service are easy to understand for everyone who uses Snapchat," a Snapchat spokesperson told the BBC.
"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."
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Onlyfans, Twitch and Snapchat rules 'impenetrable'
