You are around chemical fumes all day long like permanent waving solution, permanent hair color, etc. It is also possible while you are mixing bleach to accidentally inhale the powder in your nose which can be harmful. You can have an allergic reaction to any of these chemicals as well as stain your skin with hair color. 
You have to stand on your feet all day. It is vital that you wear comfortable shoes and stand on a mat for support. You can't always eat when it starts to get busy so you have to make sure that you bring your food with you or eat something when you get a chance. You can get hair splinters in your feet or hands and the hair from clipper cuts sticks in your clothing which can feel extremely itchy and uncomfortable. You are also around people all the time so if someone is sick there is the possibility that you can catch what they have. 
You can not always please everyone so there will be times someone might not be in love with what you have done to their hair. Also it is possible for your hair color to not come out as you expected or there is always the possibility that they have not told you everything that is on their hair and you could burn their hair off. 
You don't always get benefits and it is hard to build up clients if you flip flop jobs a lot. 
These are all absolute extremes and do not usually happen unless you are unexperienced or you are careless.