Let me set the bar, her appearances are usually from Sloane's POV so it's easy to get swept up in her rhythm but taking a step back reveals some truly dismal performance.
Starting her tour in the big leagues over Endor in The Levers of Power, Sloane is basically on probation and has only recently gotten her Admiral plaque back after being busted down to Commodore from Vice Admiral by the Grand Inquisitor for poor performance in prior media. She has received command of her ISD The Vigilance back from Vader, but she also has an ISB loyalty officer assigned who is questioning her about her decision making process and watching her for signs of disloyalty.
In the opening skirmish of the battle she for seemingly no reason orders the crew to reinforce the bridge deflector shields. This allows a short lived trio of Rebel CR-90 corvettes to overwhelm the normally battleship grade forward shields of her ISD to burn out her primary forward shield generators leaving them on auxiliary generators.
As she monitors the ongoing battle, she starts drawing up attack plans for mop up or pursuit of the Rebels, plans for dealing with B-wings, and repeatedly attempts to get ahold of ground forces control until she is interrupted by the Death Star's shield going down. By immediately switching her view from the space battle tacmap to bring up footage of the ground shield station burning, she misses the crucial moment the Rebel Red and Gold squadrons form up on the Millennium Falcon to streak out of the melee in the Imperial battle line to begin their attack run. This lapse leaves her fighter squadrons waiting for permission to engage until they are out of position and too late to pursue.
Then she recalls all of her TIEs to her hangar bays against her ISB handler's attempts to countermand those orders, trying to form up on the Executor's flank to cover it with ineffectual turbolaser fire (instead of sending fighters) until Rebel starfighters take out the Executor's bridge. Seeing the dreadnought go down she attempts to take command of all remaining Imperial forces by pulling rank. She sends out orders to all of the other ISDs to also recall their TIE squadrons to the hangars, kills her ISB oversight officer, then tells all of the ships to follow her to jump to Annaj all before the Death Star actually blows up.
Sloane justifies to herself that she's doing what has to be done to save the Empire by preemptively running, but up to this point the Imperial ISD battle line is described as holding fairly well in the brawl with Interdictor support pinning the Rebels in system. She makes no attempts to call for reinforcements and ends up abandoning the ISD Eviscerator under command of Admiral Garrick Versio (who is either the same rank or higher than her if she is back at Vice Admiral) and all other remaining Imperial forces at Endor.
She is constantly distracting herself planning about what happens after the battle, fixating on single threats, or focusing on things on the ground that she shouldn't even be focusing on given she's a fleet Admiral not a General. Reading what is written she pretty much threw the entire battle of Endor for the Empire in the canon timeline and makes a very good case for why her superiors put her under ISB review.
To be fair the loss of the Death Star and the Executor is quite a blow along with the bulk of high command, but she personally hates the Death Star as a waste of resources and if she is properly running the numbers in her head as described it shouldn’t be any reason for a supposedly steady officer to panic. Overall the loss of the Executor makes up a single digit percentage of the Empire's supercapital might (being 1 of 13 canon Executor class ships) and the handful of lost ISDs don't even add up to a whole percentage point of the Imperial Navy's star destroyer strength. The post-Endor Imperial navy is still at around 95-99% of full strength given prior large capital ship engagements up to this point have almost all been resounding Imperial victories during the Rebel's failed Mid Rim Offensive.
Her showing in the subsequent of the Aftermath trilogy is also quite poor. She arrives at Akiva under the command of Gallius Rax, fails to stop the Rebel scout Wedge Antilles from sending a distress signal to nearby Rebels, then proceeds to gaslight the rest of the other Imperial ranking officers at the Imperial Future Council conference that their location is unknown and secure to the Rebels.
When more Rebel scouts are destroyed jumping into the system she is adamant that they hide their ISDs instead of holding a protective blockade, allowing part of a followup Rebel special forces detachment of drop commandos to make landfall before the ground based air defenses can shoot the transport down. These special forces units then infiltrate the capital learning about the existence of the IFC, coinciding with a public uprising caused by yet another Rebel pilot slipping through the planet’s air defenses via a smuggler.
Moff Pandion is outraged and has been arguing to bring back his Star Destroyer the entire time but Sloane continues to make excuses such as ‘not provoking the New Republic fleet’. Her prisoner then briefly escapes and manages to send a message to said New Republic defense fleet. Sloanes manages to buy the Rebels from the local crime lords, but this is moot as the Imperials have to make her escape from the planetary uprising. The other Imperial leaders fed up with Sloane recall their fleet to orbit, while fleeing Sloane kills the local governor attempting to seek refuge with them after she destabilized his world. Sloane and the rest of the officers flee in a yacht to her ship the Vigilance which gets pummeled into scrap by the arriving New Republic fleet.
She then abandons her flagship to flee to Pandion’s ship the ISD Vanquish, killing Moff Pandion who has boarded the shuttle to dispose of Sloane and take command of the fleet. She returns to Rax, who tells her he also leaked info on the Akiva summit to the New Republic to clean house. Rax apprently really likes her, declaring Sloane competent for surviving and quickly promotes her to Grand Admiral.
Sloane spends months as Grand Admiral as the public face of the Empire, de-factor commanding all Imperial forces on behalf of Rax against the New Republic and Imperial breakaways. As the top military commander she is overseeing ongoing operation Cinder purges killing billions of civilians (unsurprising given her early appearances include commanding the forces hunting down Alderanian refugee ships). She is somehow surprised to discover that under her supreme leadership along with her boss Rax who’s entire goal under operation Cinder is wiping out the Empire for failing Palpatine, loyal Imperial forces have somehow dwindled 10% of their former strength. According to the records the Imperial supercapital fleet fell from 12 Executor class (13-1 Executor lost at Endor before she took command) down to 1 available with another suspiciously MIA hidden by Rax.
She gets suspicious of Rax (who has been leaking intel to the Rebels) and eventually tries to kill him to take control, but immediately falls for Rax's ‘trust me bro’ on his newest plan to attack Chandrilla with warcrimes to make her Empress. Participating in this plan by pretending to by Rax’s alias ‘the Operator’ who has been leaking critical intel to the NR leading to major Republic victories goes poorly for her. The attack begins with her in the blast zone again, causing her to once again rethink her life choices and go on a further investigative journey into Rax.
Her boss Rax that she's been enabling then takes most of the remaining forces of that 10% military strength to Jakku to die and the Rebels form up to clean house. She then finally decides to kill Rax after he tries to blow up the planet Jakku everyone is fighting around, then takes over the First Order contingencies to begin again.
Not only is she a seemingly incompetent commander, but she also chooses to serve team Cinder under Rax after Endor despite it actively betraying her beloved Empire. She is both directly or indirectly the cause of most of the Empire’s military losses both in her failings at Endor, and serving under the traitorous Rax all the way up to de-facto commander of the entire Imperial military as Grand Admiral.
Are there any Star Wars admirals who even come close to the disastrous amount of damage done to their own faction under their leadership besides Rax himself who was doing it on purpose?
