The advantage is that you usually earn more than most other PhD students. The qualification criteria are also less strict, so, if you have research/lab experience, you may get a place with a 2(ii), instead of the usual 2(i).
The downside is that in addition to paying tax, you will have to complete your Research Assistant (RA) duties as well as your PhD work. This depends on the attitude of your supervisor and the conditions of the funding for your position. In many cases the RA's duties are the same as their PhD work. It can also take longer to complete because of these difficulties and because you are less likely to be given paid time off to write your thesis.
If you are offered such a position then it's probably the best way to do a PhD.