The answer depends on Maro's role in the discussion. Here are two different interpretations:

(1) It's Maro's classification:

Even though we may question the accuracy of [Maro's] classification, which was later refined by Balkins, **still,** his work offers a convincing argument for the differentiation of dialects based on nominal suffix usage.

OR: Even though the accuracy of [Maro's] classification, later refined by Balkins, may be open to question, his work **nevertheless** offers a convincing argument for the differentiation of dialects based on nominal suffix usage.

(2) It's someone else's classification:

Even though we may question the accuracy of this classification, which was later refined by Balkins, **Maro, on the other hand,** offers a convincing argument for the differentiation of dialects based on nominal suffix usage.

Even though the accuracy of this classification, later refined by Balkins, is open to question, **Maro, on the other hand,** offers a convincing argument for the differentiation of dialects based on nominal suffix usage.

Mesmo considerando inacurada a sua classificação, a qual foi retrabalhada por Balkins, já em Maro [AN AUTHOR] observa-se que um argumento substancial para a diferenciação entre os dialetos estaria na base da utilização dos sufixos nominais.