I keep hearing the Vader complaint on him cutting down the Rebel Troopers. First of all, isn't this what we haven't wanted for years? We finally get to see Vader kicking butt on screen. As for practicality, in Rogue One the Rebels have just stolen the Death Star plans, but have not had to analyze them. If Vader manages to get them back, the Alliance gains nothing. Time is of the essence, hence why Vader is doing it himself. If the Rebels escape, they can get the plans to Alliance leadership. In ANH, Vader has captured the Tantive IV and has disabled it. It isn't going anywhere. The Rebels are not going to escape. He can take his time so he sends in the Stormtroopers to pick off the Rebels.