There are, of course, two answers to the question of how Orthodox is some non-Orthodox theological work or whatever, one of which is most annoyingly phrased as a question, so that's what I'll do here:
How drinkable is poisoned water? (This is one answer. Why seek the living among the dead?)
However, in other respects, there may be good mixed with the bad, and it may be possible to sort it out.
But, when you're asking about a specific work like the Summa, you're dealing with a work composed in a context different, if not sometime antithetical to, the Orthodox mindset which existed in the West prior to the schism, and also in the East at the time Aquinas wrote. So, you will not just find teachings out of step, but constructs and understandings. Not only are the answers wrong, but also the questions. Now, I'm not trying to rip on non-Orthodox works, or scholasticism in particular. But this is just what we're dealing with in the West in general from 1100 on--thesis, antithesis, back and forth and getting nowhere, creating problems intentionally, and then solving them by creating new problems. It's nuts. It represents humanism--at first with Aquinas it is human reason to understand and expound upon Divine revelation, and then it is human reason glorified to be on par with divine revelation, until eventually it becomes human reason surpassing divine revelation. While you won't find an ecumenical council dogmatizing on this, the aspect and understanding of divine revelation over and confounding human reasoning is key in Orthodoxy. So, the value of works like the summa becomes quite peripheral.