We were at Banga in April last year, out of season, Friday, Saturday and Sunday nights the locals swarm to the day visitors area at Banga and Kosi mouth, parties with drunken hordes and doef-doef till early hours of the morning. Beach and parking/braai area strewn with empty bottles and trash.
Even on the access road there were empty beer bottles every few hundred meters, my wife and I predicted that there is going to be an accident on the Banga-Manguzi road before long and just as we got home we heard that a drunk local crashed into a family on vacation.
This is not the Banga/Kosi that I remembered and wanted to show to my wife to experience! The moment I saw that the main access road was wide and scraped I realized that things has change.
The days of peace and tranquility are gone.
If you own a boat then camping next to the lake is an viable option.
Our second week was at Mabibi and its the only place that I will visit in future.
I will recommend Mabibi, its small if you don't like the big sites like Vidal or Sodwana but still safe and peaceful. They have camping (8 spots IIRC) and 3 self-catering units.
I feel sorry for Kosi and Banga, they need the visitors, but I for one will not visit there again with the nonsense going on!

Bookmarks