I'd say it's pretty good.

Now, certainly, caution is to be used when looking at the opinions of German sources in the 1950s. In "Fighting Patton", Yeide wonders why the Germans seemed to have a much more favourable view of General Patton's competence in the 1950s than during the war itself.

But there may have been other influences, as well. By 1955, of course, the Federal Republic of Germany had become an ally of the United States against the "red menace", and a member of the North Atlantic Treaty Organization. Wartime passions had faded, and perhaps a sense of good sportsmanship had emerged on both sides regarding the achievements of opposite numbers during the conflict. Westphal, in 1957, had applied for a position in the Bundeswehr and was approved by the personnel-screening board, though ultimately passed over by the Defense Ministry. Balck and Mellenthin became military consultants for NATO. Smilo von Luttwitz became a Generalleutnant in the Bundeswehr.

So, these officers are obviously writing with an eye to their own future careers in the new NATO environment. Saying bad things about themselves or the Americans or British would not be career enhancing moves.

However, in this case, we are talking about actions against the Red Army, there is nothing to 'offend' their new 'friends', which reduces one source of friction.

Secondly, if you go to the very end of the document, appendix C, the work of the group under Halder is the second-to-last mentioned source used by the US writer, in front only of personal interviews with individual officers. The paragraphs preceding indicate that written records of the time were the primary source of information used, and they certainly could not be contradicted by Halder et al when the original information was present in black and white.

Finally, this is a discussion of the 'meat and potatoes' of their job. Whatever about 'clean wehrmacht', the planning and execution of a military operation (especially against the current 'enemy') is nothing which would particularly benefit them personally or their nation to muck about with, unless they made, as a group, personal errors which they want to sweep under the carpet (and which may be identified by the original source documentation), and in which they can find solace as being pretty 'mechanical', as it were, and not politically concerning like the matter of whether their subordinates went around committing war crimes.

Ultimately, if you are going to ask questions of 'what happened' of people other than what was written at the time, Halder et al are the best people to ask. Be cautious, sure, but that doesn't mean to disregard the work by association.

I'll note, for example, that the document seems to be relied upon in this much shorter article in the Journal of Modern History. https://calhoun.nps.edu/bitstream/handle/10945/44218/Stolfi_Barbarossa_1982.pdf?sequence=1&isAllowed=y

I would look at the answers by /u/Georgy_K_Zhukov and /u/commiespaceinvader and /u/ted5298 in How was the relationship between West German commanders and soldiers and their Allied counterparts just after WW2? Was there a level of animosity? How did Allied commanders look at West German commanders who they fought a few months back?, which explains some of the background.