Darth Vader fighting the rebels himself at the end while exciting to look at was off and out of tone with what we see in ep 4.
What Lucas established with the OT in terms of jedi knights and the force and particularly how vader and obi wan behave kinda puts rogue one in an awkward position.
By this point in the chronology Vader and Obi-wan are old men. Thats why the the initial fight between vader and kenobi is so staid in comparison to their younger selves because at this point they're old men plus vader is mostly cyborg and has limited mobility.
And back in 1977 Lucas's idea of lightsaber fights was akin to broad sword fighting like the old english knights did. Thats why we see Luke wield his lightsaber like Excalibur.
Mind you the events of rogue one take place in the same week if not day as ANH...and we see vader do two thing thats uncharacteristic of the vader in the OT.
1. Attack foot soldiers himself. Again what he did looked exciting but it was beneath him. Thats what stormtroopers are for.
2. His fighting was very dynamic. Broad moves and wielding his saber with one hand while force pushing with the other.
If Vader had acted like that in ANH he'd kill Obi wan in 3 moves. So if they wanted to have true continuity with the OT vader shouldn't have done those things.
Also this point in the chronology the force is a myth. But therein lies the other issue the OT establishes thats kinda awkward for Rogue One to cover.
Theres only about 20 years between ep 3 and ep 4...Luke and Leia are around 19 when we first encounter then in ANH. Theyre teenagers. In their life up to that point the Force is a myth to them.
Luke is all "the Force? yeah right

" and Leia has more pressing real world issues to deal with so she never really acknowledges its existence in ep 4 and in Empire only when Luke literally calls to her using it. But other than that never really acknowledged it afterward. (she's a force sensitive who seems to have no use for it).
And literally everyone else in the galaxy seems to be non plussed about the legend of Jedi Knights and the Force so much so that Obi Wan can cut a guys arm off in a bar with a light saber and it barely raises an eyebrow and an officer can get snide with Vader directly about his "religion" while Grand Moff Tarkin treats Vader like his top henchman rather than some ultra badass you best tread lightly around.
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The officers in that room don't seem to be particularly fearful of Vader. In Rogue One everyone seemed a little too afraid of Vader.