Periyar was a racist
He propagated the false Aryan Invasion theory and claimed that India or Tamil Nadu belonged to Dravidian; and the Brahmins(Aryan or Hindus) invaded and imposed their religion. Well this is false and it has also been proved by Dr. Ambedkar in his book "Who were Shudras" in chapter 4.
Periyar was ani-national
Well, this is not my conclusion. This is what Dr. Ambedkar has hinted us. When Hindi was to be made as national language, Periyar's group protested it and started anti-hindi campaign. In response to this Dr. Ambedkar wrote a book entitled "Thoughts on Linguistics" where he says that anyone who do not accept Hindi as our national language is not and Indian.
Regarding Brahmanism and caste
In his book "Annihilation of caste", Dr. Amedkar has said that Brahmanism and caste are poison of Hinduism which has to killed. But at the same time unlike Periyar and his follower he recognizes that previously there was not caste system, it was varna. And Dr. Ambedkar states that Varna and caste are not only different but also opposite. And he goes on to say that varna is sensible as it is defined by worth and not birth.
Unfortunately due to "So-called Brahman", this flexible varna turned into Jati which started to become rigid and later into caste which became cruel, insensitive and in human. What is not spoken this todays caste is actually defined by Risley(Britisher). He categorized the people based on caste what we know today. Al though the fact remains that caste existed from long time and it is stupid and in human which has be removed.
Dr. Amedkar was against caste, but he promoted religion. In the book "Annihilation of caste" he says that Religion should be the foundation of society. He even goes to define how priesthood should be there is Hinduism, which is followed today by many Bhakti movements. Dr. Ambedkar reject caste but promoted religion as the most essential pillar of society.