Actually, Balarama is an incarnation of Sriman Narayana. The Srimad Bhagavatam designates Balarama as one of the twenty-four incarnations of Sriman Narayana:
In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarama and Lord Krsna in the family of Vrsni [the Yadu dynasty], and by so doing He removed the burden of the world.
At the same time, it is also true that Balarama is an incarnation of Sheshnaga as the Mahabharata claims:
And Baladeva of exceeding strength was a portion of the Naga, Shesha. (Mahabharata 1.61.91)
So the question now arises: How can Balarama be both an incarnation of Sheshnaga and Sriman Narayana?
This question is answered by User @Keshav Srinivasan here.
On the other hand, Buddha is not seen as an incarnation of Vishnu by many because his teachings are considered to be anti-Vedic even though there is no evidence of such.