بسم الله الرحمن الرحیم
In order to elucidate the issue, of note, there can be diverse reasons/evidences for rejecting him (and in order to prove everyone, there need long discussion and documents... but here I just mention some of the reasons in short, hopefully can help you).
- Ali (a.s.) and some of Sahabas protested him owing to ... (apart from
Umar's protest who ...) Ref.: ابوزرعه، ج۱، ص۵۴۴ and ر.ک: ابن قتیبه،
تاویل، ص۲۸؛ ابن سعد، ج۴، ص۳۳۲؛ بسوی، ج۱، ص۴۸۶
- Owing to the relationship/paradox between his hadiths/its numbers and
the time of ... with the Prophet. (sorry for incomplete illustration,
so, in order to have a detailed evaluating you can refer to the
source --if you are familiar to Arabic or Farsi [Persian]--)
- As another related reason, (from another aspect which can be regarded for Shia Islam too: Bukhari, Muslim, Dhahabi, Imam Abu Ja’far Iskafi, Muttaqi Hindi and
others have reported that the second khalifah, whipped Abu-Hurairah
as a result of him going too far in forging hadiths from the Prophet
(pbuh) and strongly prohibited him from narrating any more till the
end of his reign. The reason behind this can be found in the
1- His companionship with Ka'b al-Ahbar the Jew, and
narrating from him.
2- Him narrating hollow hadiths which usually
would convey what the Israiliyyat hadiths (unauthentic and forged
hadiths by the Jews or under the influence of them that cause
distortion in Islamic tradition, history and tafsir and bring into
religion fraud beliefs) conveyed, and in reality, were the Israliyyat
3- Narrating hadiths that were the opposite of ones
narrated by the Sahabas. Imam Ali (as) and Abu Bakr opposing him.
And so on.
Of note, I strived to point out a part of the reasons to reject him, otherwise there can be more reasons to elucidate/persuade it.