I have skied quite a few lakes with islands and some without. From a purely performance perspective I can't see a difference. A proper site skies great regardless. IMHO I do prefer lakes with islands. I like the extra time islands give you to futz with shorts, gloves and so forth. I also find less experienced drivers seem to get the boat settled to speed better with islands.
I don't think islands are inherently unsafe. They are big, easy to see and don't move. You can ski into one just as well as the end of the lake. I have seen two people hit the beach, one on an island and one on the end of the lake. The guy who hit the island was very, very experienced and had skied that lake and other comp lakes for years. That lake is big, nobody saw it so they don't know what he did. He doesn't remember it but he was, for some reason, on a wakeboard at the time... The other one was a woman who said she was playing with her contacts, she was a 28mph skier. One of the craziest things I have ever seen, she wanted to spin but when the boat did she kept going straight right on to the beach. She did walk away though. Two out of several decades and thousands of passes, I have seen more people hurt driving on the access roads in that span of time.