Sources for this movie

The British, like any insular people, had a fairly high opinion of themselves and their motives that isn't universally shared. Mind you, many small nations have a similar attitude even now, don't they? I daresay they have taken a greater interest in the regimental records they kept than anything any damned foreign johnnies may have scribbled (I jest). And at the time they didn't entirely trust the Dutch and Belgians, who had been French allies / vassals.
I'd imagine the filmmakers also considered the confusion of both sides dressed pretty much the same, versus French in blue, British in red.
From memory, the Allied army was very roughly 1/3 British, 1/3 Dutch-Belgian and 1/3 German (Hanover and Brunswick). That's not counting 40-odd thousand Prussians who played a major part. So, there were more Germans than anyone else.
In the battle itself, most units (of all nationalities) performed well, particularly given that many were inexperienced. The Nassau regiments distinguished themselves particularly.
The Prince of Orange had a bad press for a long time, but I think that is not generally thought now. Possibly they thought it more diplomatic simply to leave out the Dutch-Belgians, rather than paint them as hopeless (which I think was a common opinion - in the UK (given that the vast majority would have known and cared nothing) - when the film was made).
This site, http://www.napoleon-series.org/index.html has quite a lot of information that isn't quite as spitefully slanted as the page you mentioned.

It's been a while since I've seen Waterloo, but I'm pretty sure I remember Ney mentioning the Dutch, Belgians and Brunswickers. When he says that Wellington won't hold the French for an hour, or something like that. I used to play Napoleonics, and my Waterloo army was about 1/4 British, 1/4 Dutch, 1/4 Belgian, and 1/4 Germans (Brunswick, Hanover, etc).

The Dutch made a valued contribution to the size of the Allied army at Waterloo, hence why the Prince of Orange was a very senior commander despite having no tactical sense at all. In real life he was shot on the field (some suspect by his own troops) and so the Dutch government erected the grotesque Mont de Lion at the battle field on the spot where he was injured.

The Belgians, then the countrymen of the Dutch, were a different matter. Their bravery wasn't doubted per se, but they had been fighting alongside the French before 1812 and their willingness to go into battle against their former comrades was suspect to say the least.

Moder German authors have also tried to point out that Waterloo was essentially a holding match whilst Wellington waiting for Blucher and the Prussians. This is true-without the possibility of a Prussian relief there would be no Waterloo as we knew it and Napoleon may have been the first King of a united Europe!

Wellington certainly wasn't portrayed as an a'shole as you say. Note the scene where he spares the life of the looter, the calmness with which he commanded his officers and his good humour. Besides, Wellington was renouned for being self-obsessed, and was particularly sensitive about any possible suggestion that he was Irish (he was born in Dublin).

It's rather slanted toward the British/French perspectives and pretty much the only mention of anything 'Dutch' is when they remark "I doubt that Bijlandt's brigade will stand." and later "Bijlandt's brigade has broke. Plug that gap if you please."

Agreed completely, the bad press that the Dutch get is really in need of re-evaluation - as does the somewhat inflated British view. Just that one incident, the attack of D'Erlon's Corps, was more complex than even a simple retelling of history lets on. Some accounts speak of British regiments losing formation in the confusion and definitely the Gordons got themselves into a bit of trouble - hence Picton's dying cry, "Rally the highlanders!" I read a very good and quite believable retelling which saw the Dutch regulars RETREATING but not routing certainly as has been previously implied. The movie really doesn't give the impression of 'an infamous army' of many nationalities that Nosey bemoaned he had.

Orange is something more difficult to pin down. He certainly was rather inexperienced and there is that matter of a KGL battalion which either he or German general Karl (Charles) Halkett ordered into line formation despite the presence of nearby French cavalry. More unbiased study and information would certainly be welcome.

Cheers,
Tom516