The problem with coursework is that the teachers give you a mark there and then, and use last years grade boundaries to predict what grade it is and that's not often made clear. There can be significant difference in grade boundaries and therefore UMS scores the following year. Not only that, but teachers can often mark leniently. I don't think I've ever had a piece of coursework marked that hasn't changed by a mark or two.
The UMS scalings on these lower mark modules and also internally-marked modules can also be crazy. The difference in UMS between 41/50 and 44/50 on AQA Physics 6T is a gaping 9 UMS marks, for example.
One thing I learnt from GCSE and took into A-level - Never rely too much on the coursework. If you're looking for a particular score in the exam because of what you got told about the coursework, then aim even higher.