There are two problems with kafi.
Ancient shia scholars regarded it as 100 percent authentic. Even Kulayni claimed this
“…You wanted to have a book which would be sufficient (for your religious needs) (kafin), which would include all kinds of knowledge (’ilm) of religion, which would be adequate for the student, and to which the teacher might refer. Thus it could be used by anyone who wanted knowledge of religion and of legal practice (’amal)according to only sound traditions (athar) from the truthful ones (the Imams)…Allah, the Most Majestic, the Most Gracious, has made the compilation of the book that you had wished for possible. I hope it will prove to be up to your expectations”
(source: Al-Kulayni in his Preface of Al-Kafi)
But later shia scholars were ashamed of strange stories in kafi, so they tried to evolve an ilm e rijal for these ahadith.
The second problem is shia ilm rijal.Shia have no authentic book of Rijal, all their books are corrupted. And Shia has some strange rules of acceptance of ahadith.
1)To shias,anyone who fought Ali is a kafir,so kharjis are kafir.But kharjis are thiqa ravis in shias
2) If a renowned thiqa narrator narrates from a weak or unknown narrator, some scholars will accept that hadith and some reject it.
3) Imam Jafar narrates ahadith from Prophets but he was not even born then.