Perhaps I should clarify. I didn't have a problem with Picard not knowing Risa's customs. My concern was the episode seemed to imply Picard had never even heard of Risa at all!
Let's face it, storytellers change their minds sometimes, or different storytellers reinterpret things. In that episode, it was necessary to explain what Risa was, so they treated it as something new to Picard. But later episodes kept revisiting the idea of Risa and elevated it into the Federation's best-known recreation spot, creating the continuity glitch. It's something that happens in a lot of series fiction -- the characters' knowledge of a thing reflecting the state of the audience's and writers' familiarity with it, so that something initially presented as unfamiliar becomes overly familiar with time. Like the glitch about Professor Moriarty in the Sherlock Holmes canon -- when he was introduced in "The Final Problem," Watson had never heard of him before, but when Doyle then wrote The Valley of Fear
, set before "The Final Problem," he had Watson already be familiar with Moriarty.
We can demand perfection all we want, but we're not going to get it in any human creation. Any long-running series will have continuity glitches. That's just the way it is.