It's limited probably so it doesn't take up too much space in the Today section (given that a lot of people will have other widgets there too). As for the Reminders app, you can use the scheduled view to see things organized by time (it's the little alarm clock icon on the top right). Alternatively there are some other apps that can be used that work with Reminders and have widgets, like various calendar apps like Agenda+ or Calendars 5, or some reminder apps like GoodTask, for example.