Siborne was originally gathering evidence for his meticulously constructed models, and that is what his data gathering was initially for. His history grew out of the narratives he wrote to accompany the model.
He was also writing whilst most of the senior participants were still alive and he himself remained a serving officer. The conventions of the time were that people were very protective of their honour, duels were becoming less common but were not unknown! As a result, any criticism is very circumspect. Some have accused him of being paid to favour certain officers, but objective examination of the evidence do not seem to bear this out.
Most serious students will go beyond his work to the primary source material, currently held in the British Library. This are available in more accessible format, thanks to his son "Waterloo Letters". Siborne did receive letters in German from many KGL officers and one or two Prussians. However, it is unknown how much German he read. These letters have been made available by Gareth Glover as his "Letters from Waterloo". Gareth has gone on to unearth further British and German primary sources in his Waterloo Archive series of books.
He made some errors, for example we now think that Bijlandt's brigade was withdrawn to the ridge before D'Erlon's assault. However most Anglophone authors since have largely followed the broad sweep of his narrative to some extent or another.
Like many works written in the 19th Century, you have to remember that standards of historical enquiry were lower, and national bias and prejudice was considered normal. The litery style was also quite florid and verbose, so it;s heavy going in parts. As long as you read him with all that in mind, it's actually remarkably good.
However, I'd consult more modern works, such as those by Glover, Dawson or Hussey for more objective views. To take a broader view from other nation's material I'd recommend Muilwijk, Franklin and Field.
Just a personal view, of course, but I don't think any serious Waterloo enthusiasts bookshelf is complete without it.