The simple answer: yes, all mouse actions are achievable with a graphics tablet and stylus.
More sophisticated answer: All mouse actions are achievable, but not all are equally easy.
Easier with tablet:
Drag-and-drop, box-select, all graphics / image editing work. If your pad has macro buttons or programmable areas, some routine tasks can be made easier with well-chosen macro controls.
Easier with mouse:
Precise clicking on small areas. Any action-based computer game (they're really not designed for tablets). Some tasks which need frequent mouse/keyboard switching - I find it easier to go from keyboard to mouse than to pad. (This is personal preference; try for yourself).
Scrolling can be a tossup depending on your pad; recent Wacom's have a pretty good programmable wheel, but I find mouse slightly easier.
Tablet/stylus uses very different hand motions and postures than the mouse, so it can help with some wrist injuries even if it's no easier to use. A good tablet isn't always cheap, so try before you buy if possible - some people love them, others can't use them.