Natural Sciences refers to a large number of different courses with different structures. In general, the only true thing that can be said of all of them is you don't study just a single science. However, this is arguably true of many single honours sciences (e.g. you will study some chemistry in many bioscience degrees).
At Cambridge you study maths and three sciences in first year, then three from a widde range of subjects in second year (which usually will be chosen to form an intellectually cohesive combination) and then specialise in a single subject (normally) for third and/or fourth years. At other universities, such as I believe UCL, you normally study your subjects as a major/minor combinations. For others, like Southampton, you get given a very large amount of leeway in what subjects you choose, but this is supported by core modules which consider case studies/current topics of research from different scientific perspectives.
You need to look at each course individually to understand the structure and see if it's suitable and/or relevant to your interests. As far as after you graduate, it's generically as good as any STEM degree, however depending on your options may or may not be more specifically appropriate to certain roles e.g. if you didn't specialise in biosciences you probably would not be any more qualified than someone with a physics degree for a biological lab technician position. How good it is for research (e.g. PhD etc) depends a lot on what you actually study; it can be better than single subject sciences for certain areas, depending on how you specialise (e.g. combining both biological content with physics for going into biophysics) but may also be less suitable due to not covering as much core content in one area.