The main challenges that I have is that quite often as another user mentioned we are seen as the ‘safety net’ and perhaps this means that the work can be sub-standard.
Also as I work for an agency my time is paid for by our clients and everything has an estimate. If a piece of work has a lot of bugs in it then more time is needed for a second or third round of testing but this has a knock on effect otother projects that I am booked on.