I will admit up front that I have never really learnt through the entire sefer Chafetz Chaim in depth, and that doing so would be a good thing.
However, I recently started to do so, and I think the following observation has some degree of accuracy to it.
The big problem (IMHO) is that the sefer Chafetz Chaim, and the laws of lashon hara in general, lack the diachronic and synchronic debate and development that occurs for almost all other halachot.
For example, take a particular din in kriat shema, whatever it may be. The chances will be that it comes from a particular Bavli and there may or may not be a parallel Yerushalmi.
That source will be taken and interpreted not just by Rif, but also by Rambam and Rosh. And Rif, Rambam and Rosh may disagree as to the meaning of that statement, how to balance that statement with other statements in the gemara, and whether that statement ends up being lehalacha. Each of these works (Rif, Rambam, Rosh) has supercommentaries, with competing ways of understanding their words. And upon the Rif is the Baal HaMaor, who argues on the Rif, and the Ramban, who attempts to defend the Rif.
Then we have Tur, with Beis Yosef, Bach, and Darkei Moshe each providing their own, sometimes differing commentary. Then, the Beis Yosef, after writing commentaries on Rambam and Tur, wrote the Shulchan Aruch, those we have the Rema giving another view side by side.
Upon the page of Shulchan Aruch, we not only have Shach, but we have e.g. Taz. And while later we have the the Chafetz Chaim reworking and harmonizing past material into the Mishnah Berurah, we also have his contemporary who wrote the Aruch haShulchan.
That these halachot of kriat Shema were organized systematically, and received their own simanim and seifim in Shulchan Aruch, means that they were touched upon and debated by multiple generations of Rishonim and Acharonim. It has been highly processed. And in any particular din, we can read through many different perspectives on that din and come to some conclusion.
Contrast that situation to that of the dinim in sefer Chafetz Chaim. Sure, he bases himself on gemaras and various rishonim. But these dinim, with their particulars, are resource-poor, such that we do not get the type of in-depth debate and development throughout generations. And where is the Chafetz Chaim's bar plugta? We would normally have Aruch haShulchan, but Aruch haShulchan did not (afaik) write as extensively on these halachos.
Looking at particulars, I can see certain assertions which I think would be questioned. And Mishnah Brurah has a certain derech of satisfying multiple positions that others would not necessarily go along with. I won't go into the particulars here. It does not really matter; that such debate would have come up is almost certain. For why should these halachos be any different from halachos in every siman in Shulchan Aruch?
A certain sophistication comes about from this debate and development. And while the Chafetz Chaim wrote for his time, certain day-to-day situations come up that require application of principles to "hairier" situations. And people can readily misapply these rules, as is bound to happen when one takes common sense and formalizes it, and when tries to apply rules to extremely complicated situations with many factors.
I think that he needs a bar plugta. I was thinking that it might eventually be a project of this blog, not just translating and expanding on Shmiras haLashon or Chafetz Chaim as others do, but analyzing the sources and arguing with them at times. Of course, people's reaction may well be to question who this Josh fellow thinks he is, and what chutzpah to do so! And for various other reasons, those capable of doing this who would be accepted likely would not dream of taking up the task.