if it is a renowned group .. then there are chances that you will be treated well. otherwise... it is advisable that one should not start working on appointment letter only , unless visa processed.... as it is illegal to work without resident visa.
Every case has his different reasons, different circumstances. If u want to know about law please study labour law of UAE. Law no 8 of 1980 UAE labour law. Check the labour law section of this website.
You can apply once more. If u don't get success . Then have to go to court.
Visitors to the UAE on all types of visas must obtain health insurance for the entire period of their visit. This insurance can be obtained from companies operating in the UAE or abroad.