They treat their part time staff like cheap casuals. They don't hold their agreements and the harassment I have witnessed on two sites was appalling. Don't even think about trying to stand up for your rights either. As my manager explained to me: "You are just a number, they'll get rid of you". They also make you do training in your own time, completely unpaid and you will face disciplinary action if you don't.
You will meet some genuinely nice people.
They will break every agreement, Their whistleblowing service is just a glorified messaging service for upper management, If you become a problem they will force you to quit or just fire you.