1- Sunni hadith is not directly from the companions. Most Sunni works of hadith, such as Bukhari and others, were compiled some 200 years after the Prophet (s). From Bukhari to the Prophet, you have more than two centuries. We believe many fabrications and deviations (or inaccuracies) happened during these 2-3 centuries.
2- Shia hadith, even though we don’t claim it’s all correct (we still need to research every hadith) is more reliable than Sunni hadith because it has a shorter link to an infallible. We believe the Imams are infallible. The Prophet said the Qur’an and his family never separate, and since the Qur’an is infallible then his family are also infallible. So during the era of Bukhari, Shia narrators were narrating from the Imams. So they didn’t have a 200 year gap from an infallible. We believe the hadiths of Imam Sadeq (a) are 100% authentic. If Imam Sadeq says something, he has a direct link through his infallible fathers to the Prophet. So if one narrator narrates a hadith from the Imam, he is narrating directly from an infallible, whereas Bukhari’s narrators have a long way and error-prone way to get to the Prophet.
This is one main reason why Shia hadith is much more reliable and authentic.
The evidence that our hadith is from the Prophet and the Imams is the following (as an example):
1- Kulayni gathered 16,000 hadiths in the book of Kafi
2- We research the biography of Kulayni and we come to know he was very trustworthy and reliable.
3- He mentions the source of every single hadith. He gives us the name of the narrators. We research about those narrators. If they are reliable, then we are confident the Imam said the hadith