Hans urs von Balthasar is not a good theologian. He doesn’t seem to be technically a heretic, but I know of at least one of his teachings which are in fact what theologians call “proxima haeresi” or next to heresy, i.e. something that is contrary to a teaching that has been formally revealed yet not specifically proposed by the Church for belief. There are several other teaching of Balthasar which theologians and the Church would call “erroneous,” which is more serious then it sounds.
If you’re interested in theology, I recommend you read a Thomist. Since there is not much thomistic literature today, there are plenty of used books you can get from
abebooks.com.