The thing that bites a lot of naive protocol designers who use UDP is that it doesn't guarantee order either. So you can get delivery out of order, and that gets more likely the more your packet crosses IP subnets. In part because some folks do traffic shaping, in part because UDP traffic is considered "less important" by a lot of ISPs, and in part because new switches, like the latest compilers, have enough cpu in the control plane to play games on packets flying about.