I have that identical unit.  I wanted the DR, but the Swisher was cheaper and I needed it badly.  Later I bought a used DR at auction for very little money, so I have both.  I have used these units in very heavy stuff, and not had problems.  Yes, the heavier the stuff the faster the string wears out, but I have never had it break off as you describe.  

Based on my experience, I would say you just have poor quality string, and there is a lot of poor quality string available.  I order mine from DR, and use their heavy duty string, they have different grades.

You may wish to experiment with small amounts of different material before giving up.