Tarkin believed that Krennic was partially at fault for the breach on Scarif:
From Tarkin's Point of View:
Director Krennic, General Ramda, and Admiral Gorin had all failed to solve the problem at hand, granting the rebels opportunity after opportunity to seize the schematics from the Citadel.
It was a show of incompetence so great that Tarkin was almost curious to know how Krennic might explain it away.
Rogue One: A Star Wars Story, Chapter 21
But Krennic's position is closer to a supervisor of military projects than a commander. His life is outlined in Catalyst and the Rogue One Ultimate Visual Guide:
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First, he studied Architecture at the Futures Program:
When Krennic was a young man, his brilliance earned him a place in the Brentaal Futures Program, an educational institute meant to incubate developing prodigies. Young Orson's listed area of excellence was architecture.
Rogue One Ultimate Visual Guide, page 108
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Then, he joined the Corps of Engineers, reconstructing Coruscant and supervising construction projects:
During the Clone Wars, it was Krennic who oversaw the transformation of municipal grounds on Coruscant into military command centers.
Rogue One Ultimate Visual Guide, page 108
... he had risen quickly to the fore in the Republic Corps of Engineers, supervising the construction of enormous projects ...
Catalyst: A Rogue One Novel, Chapter 6
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Next, he was transferred from the Corps of Engineers to the Strategic Advisory Cell, requiring him to become a weapons expert:
Recently transferred from the Crops of Engineers to the cell's Special Weapons Group, ...
The new assignment had also required a return to school to suffer through intensive courses on the weapons Rothana, Kuat, and others had built for the Kaminoans to equip the Grand Army, and on the weapons Baktoid, Hoersch-Kessel, and others were engineering and producting for the Separatists. On graduating he had supervised a brain trust of research experts who reported directly to the upper echelon of the Republic military.
Catalyst: A Rogue One Novel, Chapter 3
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Finally, sometime after the Empire takes over, the Strategic Advisory Cell is absorbed into the Tarkin Initiative, and Krennic becomes Director of Advanced Weapons Research:
... [Krennic's shuttle] is on Tarkin Initiative business, with an Imperial advanced weapons director aboard.
Rogue One Ultimate Visual Guide, page 24
I doubt that the change to the Tarkin Initiative required knowledge on military tactics, as the Rogue One Ultimate Visual Guide describes the initiative as a "think tank" (page 127). Therefore, Krennic never received any ground tactics training, nor was he particularly interested: the Desonesto One: Uma História de Star Wars novelization notes that "Krennic was not, at heart, a military man; he believed that if a battle had to be waged, something had already gone wrong" (Chapter 18).
Therefore, given that Gorin was in charge of the star destroyers and Ramda was in charge of the garrison, would Krennic actually be at fault for the breach or was Tarkin's perspective influenced by the rivalry between the two?
[Note: I don't think Tarkin's view has anything to do with Galen because the grand moff is only considering "the problem at hand", implying that Krennic's failure has to do with the rebel assault.
In addition, when Krennic commands the officers on Scarif, it is because of their gross incompetence - when the attack happened:
The officers were yammering behind him. He heard no words, but recognized a shared tone of surprise and confusion. Were they truly so oblivious?
And after he shouts "Are we blind?":
He had the attention of the room. Attention was not what he required.
Therefore, Krennic only stepped in because nobody was reacting properly. ]