Are there rules? That’s up to the show chair. There are both guidelines and conventions, but I have yet to find a good resource for this in print. I’ll have to write more about it. One common example: accents look good when there at least three different plants growing together. Another: accents look best when they are full an lush.
Are they less complex in Japan? Depends on the show. Their use is highly conventionalized in the Kokufu exhibition (many pyrrosia). Many are similar to each other, but they do look good. Other exhibits can feature more creative approaches.
Can they be a distraction? Definitely! I like when they provide contrast and indicate the season or place in which the bonsai grows.
Here are some accents displayed at an exhibit in Texas last year: