Imperials have all the tools they need to win.
In general they just rely too much on the highly fickle green dice.... this lack of consistency means they can struggle to win 8-9 games out of 9 you need to win most events... but I would guess (unconfirmed) that they win a reasonable proportion if games in total.
So for the 5% realistically trying to win a major event they are a risk... and not faction of choice. For the rest of us, they are just fine.
And vader is cool... you know he is!