A lot of people say Russell’s History is unreliable, particularly with regard to German idealists, with whom he was unsympathetic, but I think it is Hegel, much more than Kant, that he is supposed to have got wrong. Furthermore, Russell was something of a pioneer of the study of Leibniz, who, in a sense, was the founder of the German idealist tradition, but who was almost forgotten in the English speaking world before Russell revived his reputation. (No doubt our understanding of Leibniz has moved on a lot since Russell’s time, but if not for Russell, that process might never have got going.)
Russell’s own philosophy, however, was a pretty direct and vehement reaction against the British idealists, people like Bradley, Green, and McTaggart, who, in the late 19th century, had taken over and carried on the post-Hegelian tradition. Thus Russell really had very little sympathy for this whole genre of philosophy. Kant, however, really only had one foot in this tradition (or, rather, German idealism flowed out of one side of Kant’s work), and another in the Empiricism that Russell greatly admired, and saw himself as reviving. I rather think he thought that Kant was a truly great philosopher, but one from whom Hegel and the other idealists had taken all the wrong lessons.
For all its faults, though, Russell’s History is well worth reading, and almost certainly a much better, livelier read than most attempts at a comprehensive history of philosophy. If you are a beginner in the field, it is still a great place to start, provided it isn’t also where you end. Any such book, of such ambition, is bound to contain many biases, errors and inaccuracies, and to gloss over a lot of details (and even to completely leave stuff out), some of which might be considered very important by other scholars. The thing is not to not read Russell, but to read it critically, and not to take it as the last word or the definitive framework, whether on Kant, or Hegel, or anyone else, or, indeed, on which philosophers belong in the canon (opinions about which can change a lot over time).
Doper Half Man Half Wit seems to be very knowledgeable about the German philosophical tradition, and can probably tell you in some detail just where Russell got it wrong.