I think the complete grouping system should be redone. Apart form the many bugs written earlier in this thread, units when grouped often attack the wrong enemy unit, most of the times they pick enemy units closest by, instead of the unit you click on.

Ordering a group into the far distance by right click and draw, often results in the game getting stuck, especially when you get close to minimap or command icons. To avoid that you need to zoom in first on the spot you want the unit to go, then give the order. Rather annoying.