Again the theological proposal is solid, and the majority of the quotes used are authentic. Unless you can refute the theological arguments that he presents, it is to be upheld as an extremely valuable source.
The problems with the work run deeper. Aquinas himself wished to cut through the linguistic chaos in an attempt to show the doctrinal harmony between the Greeks and Latins (in other words, his primary argument tended towards saying that the Greek teaching was identical to the Latin teaching, but that on account of linguistic issues and the obstinacy of the Greeks, the Greeks could not see this). The problem is that Contra Errores Graecorum fell into the same linguistic problems that Aquinas was attempting to resolve, because the work often conflates the meaning of several Greek verbs which had developed distinct meanings amongst Greek-speaking Christians.
That’s also definitely a problem. Although St. Thomas is aware that there are differences in usage of mostly synonymous term between the Latin and Greek writers, he was still working from Latin translations. I suppose a chief example would be that the Latin texts he consulted naturally translate proienai/ekporeuomai alike as “procedere,” since that technical distinction was foreign to the Latin tradition. On the one hand, this does not entirely thwart his project as far as examining the patristic texts, since it is not clear that any of the authors quoted was working with such a terminological distinction explicitly in mind, especially as it came to be understood among the Byzantine theologians contemporary to St. Thomas. On the other, his work fails to effectively engage the Byzantine arguments since his understanding of his opponents position is somewhat superficial.
I hope this answer helps you
The Holy Spirit proceeds (sent) from the Father and the Son (filioque)
Dual procession of the HS i.e. Dual sent is orthodox teaching.
Dual source for the HS is heretical
dual procession ≠ dual source