They are often very knowledgeable about the topic that they teach, but are absolutely incompetent when it comes to teaching it due to their inability to speak to speak English fluently.
They are often overworked by the entire department for which they are employed by. Often times they have their own coursework, projects, and research to focus on but since they need the money to stay in grad school they agree to help with coursework. Since they are essentially at the mercy of the department, it's likely that they will take their frustration out on your grades and you will most likely not do very well in the courses.
Two days later
Student: WTF, I thought I was gonna do well. My TA is such a jerk.
Student 2: Please change my grade, I'll do anything.
TA: Sorry, I wouldn't have time for anything....
Student 3: If I don't get an A in this class, I'll get disowned by my family.
TA: I was disowned the moment I graduated college, it's honestly not that bad.