People have been complaining about the YouTube mobile experience for years. It's just not the same as it is from the web. There are certainly plenty of similar complaints for numerous other popular sites and apps, but YouTube's mobile apps have developed a reputation for just not being as great as YouTube itself.
The search feature from YouTube's mobile apps, specifically, will often not deliver the same results as the YouTube desktop search feature.
Google and the YouTube know the mobile experience doesn't match the desktop experience, and a YouTube product manager has weighed in on the topic in a discussion on Quora.
One user asked, "Why is Youtube's search from mobile devices so inferior to its web-based search?"
YouTube's Matthew Darby responded, "Mostly, because not all content is available on all devices. Content owners are given the option to exclude content from some platforms, particularly when monetization is not available. This often means the video you're looking for isn't available on that device, and so it's removed from the search results you see. The most obvious example is iOS, which doesn't have a lot of content available for this reason, including music videos from the major record labels."
Update: A source close to the situation tells WebProNews that this response is not totally accurate. For example, the source says, there is major label music video content that is accessible via mobile devices.
Sure, iOS is indeed an example, but plenty of Android users have complained as well, so let's not turn this into a Google vs. Apple thing. Googling "why does youtube mobile search suck," the first result is "Does anyone else think the Android YouTube app sucks?" from androidforums.com.
"For some reason when I search stuff on YouTube.com I get much more results than I do on the mobile app," the user who started that thread complained.