Voltage drop is only an issue when you're powering a device by USB, really. But a longer cable and joints can also cause difficulties with data transfer at higher rates. You have issues with electromagnetic fields, capacitance, and so on. There's a FAQ on the subject
here, which says:
Q2: Why can't I use a cable longer than 3 or 5m?
A2: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.