Basically what zircon said.
Also you have to keep in mind that OCR looks for single, cohesive pieces. Sometimes people want to pay tribute to a game so they try to pack as many themes from the game into their piece at a time. What ends up happening is that their piece ends up sounding like a string of quick remix sketches, rather than something that works as a single piece of music with a beginning, middle, and end.
One crit I've used a few times when judging medleys in the past has been, "I really like that 20 second bit where you covered that one theme; I wish that was a fleshed out remix." Often I feel like it's a wasted opportunity. As soon as I start getting into the idea being presented, the song switches gears to something completely different. It feels like a cop-out sometimes; the artist had an idea for how to interpret a song, but they didn't want to sit down and really develop that idea, so they just do a 20 second tease or something.
Medleys aren't bad or wrong, and I don't really like calling them lazy either, because they do take some work, but for OCR, you want your submission to be an idea, not a string of ideas.
I've done some mixes with multiple sources (usually only two though) and my approach is always to use the changing of source tunes as a way to provide a change of pace; the second source essentially becomes a B-section for the piece which will callback or lead itself back to the A-section (the first source). Regardless of the number of sources you use, callbacks are a great way to keep things cohesive.