I lived in Germany for a few years, and got to spend some time on the north sea islands, Juist was my least favorite. One thing I will say Juist has in its favor are seals, lots of seals, but otherwise there is very little to do. I had a really bad time because I had some kind of strange reaction after swimming in the sea, and 30 minutes after I got out of the water, my feet and legs swelled. I couldn’t wear shoes for a month and could barely walk, and nobody had an answer for me as to what it was, and since cars aren’t allowed, I had to walk 2 miles to the only town on the island.
I won’t bash Juist, because the people were all mostly nice (except the evil witch that runs the youth hostel and hates youths) and the wattwanderung is a lot of fun, they take you out when the tide recedes and show you all the sea life that was left behind. You get to see a lot of crabs and happy seagulls, and the best is when they take out the mussels that are as big as a fist. It’s informative, but fun.
Just be careful in the water!