So far they are legit and there are indeed accusations but they have been able to resolve the accusations so far, and how they continue to develop by becoming a sponsor for one of the English premier league clubs proves they are legit and serious about the business they are running.
Regarding the max limit, I don't know for sure because so far I only use a small average bet.
Withdrawals at Duelbits are processed quite quickly, whether it's instant or not I don't know for sure because when the casino has processed withdrawals from its customers, network density will greatly determine how long our coins will be credited in our personal wallet, if the network is congested it will certainly take us longer accept the coin so the customer must also understand this.