What is the ruling regarding Hadith Rejecters?

I'm looking for Sunni view. Please mention if Shia view is presented. I have seen people who outright rejects Hadith and says that a lot of changes have been made to the Hadith, so its it not reliable anymore. They claim that since only Quran is the word of GOD and promised to be saved, then how can someone claim that another text is authenticated? Some people call them Quranists who do not accept any and all Hadiths and believe that the whole Shari'ah can be understood by studying the Quran. We also have some other sects who only accept Siha e Setta (Six Authentic books) of Hadith. They are the most popular books of Hadith. If reference from any other book is presented to them, they reject it and say that if it doesn't exist in these six and specially Al-Bukhari/Al-Muslim, they will not consider it. I have two questions, but since both are very similar in nature, I will ask them here together. Q1. Why do Quranists reject all the Hadiths? How can they extract many rulings/understandings that our Ai'mah have extracted using Quran o Sunnah? In case they are rejecting half of the Deen, what is their ruling? Are they out of the folds of Islam or not? Example: How can they prove 5 prayers and number of rakats which is discussed in Hadith when they don't even accept them? Another example, Quran only discusses about some Haram Animals, how can they then decide which animals are haram and which halal when they don't accept Fiqh and Hadith O Sunnah. Q2. Why do some sects reject Hadith from other books? Are those books not reliable? Is popularity the only standard for them to accept Hadith from? Aren't there Authentic/Sahih Ahadith in those other books? Is this sect out of the folds of Islam or not? As they choose what they like and leave what they don't? They follow their 'nafs' while claiming to be following the Quran o Sunnah but when confronted with Hadith from other books, they retaliate with "No, this Hadith is not in Bukhari, so I will not accept it and it is not authentic. It is weak." For reference, see this wiki-link to see names of other sources of Hadith: List of Sunni Hadith Books