He was absolutely not a coward, nor was he incompetent. He was daring, and impetuous like any 23 year old commanding men would be. He made mistakes and launched costly assaults which certainly stain his record as a military commander. But the portrayal in Sharpe's Waterloo is definitely not accurate.

So William of Orange's career started in the Peninsular war, but he only served as an aide de campe to Wellington and so we can't really make any actual judgements about his military record with that regard.

His actual military career started at the battle of Quatre Bras, a battle that preceded Waterloo. The battle was over a crossroad that would have allowed Wellington to march and meet up with the Prussians under Blucher currently fighting Napoleon at the battle of Ligny. The important crossroads were held by the Prince of Orange, who because of political reasons had been given command of I corps in Wellington's Anglo-Dutch army. Wellington had ordered I corps to march away from Quatre Bras to a new position. The Prince of Orange and his chief of staff decided to ignore that order. Eventually they came into conflict with Napoleon's Marshall Ney, who had been ordered to seize the cross road. It eventually turned into an all out battle as each side sent in reinforcements.

In this battle the Prince of Orange generally conducted himself well. His men held the line being slowly pushed back by the French as they poured more men in. Eventually Wellington came to take command and Orange decided to launch a counterattack with his cavalry, it didn't work and was repulsed with heavy losses (like I said impetuous). Eventually the battle was won by the Allies, but they were not able to march to the Prussian's aide. So in Orange's first real command he disobeyed an order(or at least his chief of staff did), but ultimately it was smart to do so and he arguably helped save Wellington's position.

Moving onto Waterloo itself. Orange again basically acted as expected. He was one of Wellington's commanders and did his job dutifully. But Wellington's command style meant that he often moved men himself, without bothering to consult with his subordinates.

He did however make more questionable decisions that really leave a black mark on his reputation.

First off when Wellington's men had formed into squares to repulse a French cavalry attack, Orange ordered one of the squares to attack a French cavalry unit that was forming up to attack. This worked and drove off the cavalry. But the problem was it exposed the square to cannon and rifle fire, as you can imagine men in a very dense squares are easy targets. It caused horrendous losses. In Sharpe's Waterloo this is the event that causes Sharpe to try and kill the Prince of Orange.

The next major issue was the recapture of La Haye Sainte, a vital farmhouse that anchored Wellington's position. A colonel by the name of von Ompteda was ordered to re-capture it from the French. He objected because he didn't have enough men. The Prince of Orange stepped in and ordered him to advance. Von Ompteda's men would later be cut down almost entirely.

However he did almost redeem himself late in the battle. The Imperial Guard of Napoleon had attacked the centre of Wellington's army and had almost broken through. They sent the initial British units into a route. A British major named Macready said:

"I cannot conceive what the enemy was about during our confusion. Fifty cuirassiers would have annihilated our brigade."

The reason the guard couldn't exploit the breach was because the prince of Orange had led a bayonet charge against the Guard. It didn't damage the guard all that much, but it did delay them. Orange was wounded in the shoulder and the British had time to reorient their men and repulse the guard. It was a costly attack by Orange but it prevented the guard from capitalizing on the British route.

So yeah. He wasn't an idiot, nor can he really be called a coward, but he wasn't a good commander either.

Cornwell's work is very, very light weight formulaic fiction. His books always include variations on the same formula of featured characters, the dastardly villain and the effete, cowardly and incompetent noble officer are two of them. The Prince of Orange is the effete etc character in this particular novel. The comments below establish the reality.