Some scholars think that the Vishnu Puranas are actually tamasic, but what are their arguments for this?
This is based on what Brahmaji said in Rudra Samhita of the Shiva Purana itself which claims to be the greatest of all Puranas. Vishnu is stated as only a demi-god on the same level as Rudra. While Shiva is free from all Gunas. And Vishnu is not only on the same level as Rudra, he is called Tamasic. Also Laxmi is called Tamasic.
Vishnu, Rudra and I (Brahma) represent the Gunas.
Siva is free from Gunas. He is the Supreme Brahman, the undecaying.
Vishnu is of Sattva attribute, I (Brahma) am of Rajas attribute and Rudra is of Tamas attribute. This is only in the view of activities in this world. But in fact in name it is otherwise.
Vishnu is Taamasik nature within
Goddess of speech is Rajasic nature; Sati is of Saattvika nature and Lakshmi is of Taamasika nature; the great Goddess Sivaa is of three natures.
So this naturally means that Vaishnava puranas are glorifying a tamasic demi-god and goddess if we believe this. There is also a major Vaishnava purana (Narada purana) which claims that a Shaiva purana (Linga purana) is the greatest. This is given in Chapter 102:
It indicates the greatness of Hara. It is greater than all the other Puranas. It is the essence of the three worlds.
This and some other points I'm not mentioning here are taken as evidence that Vaishnava puranas actually accept the supremacy of Shiva and are contradicting their own statements so they are proved to be tamasic / full of ignorance.