Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answer updated 3 years ago

This is a complicated question, since Muslims of differing sects have differing opinions about what hadith narrators are deemed acceptable, or which hadith are deemed acceptable.

For this reason, I feel it is best to let the content speak for itself, and so I invite you to read Sahih Bukhari and Sahih Muslim carefully. Do you feel that all of the narrations reflect a deep, dignified set of teachings about God, ethics, or the Prophet (S)? Do you feel that all the narrations are in accordance with reason and an enlightened worldview?

Certainly, some of them are, but you only need to find one or two that are not to suggest that they are all not.

For instance, might I point to a narration in the section on the "oneness of God" (the final chapter), in which the Prophet Sulayman (A) has relations with 60 wives in one night. This is of course his personal business, but is it really realistic, respectful, or necessary to even mention it? And, what is this doing in a chapter on God? Yes, there is a point to the narration (that one should say "if Allah wills" for everything), but this is hardly a serious discussion about the nature of Allah. In fact, most of the narrations in this chapter are rather shallow and do not really add much to our understanding of the nature of Allah apart from some surface level things. 

If, after reading Sahih Bukhari and Sahih Muslim, you feel that they are all transmitted from a man of God, or on behalf of him by his close supporters, that's fine and is your perspective. After all, there have been plenty of Muslims historically that embraced these books. I am not of that view, but that's part of why I'm not Sunni. Allah in the Qur'an encourages us to think.

(However, here is someone's work on the topic which you could consider:

As for al-Kafi, there is no need to consider it all as true. However, one could say that it contains truth and that much of it is true. Anyway, if you take one subject - say, the oneness of God - and compare the sections of Sahih al-Bukhari and al-Kafi, you will find a significant difference in terms of the depth of the discussion. But, I leave it to you to do the comparison.