I haven't even been to university yet but,
based on my brother's experience, cambridge degrees are significantly harder. In fact, the final year determines your degree classification, the previous 2 years' grades are not taken into consideration - that makes it harder in itself. Many other unis give weighting to the first 2 years.
My older brother got a 3rd for his maths with physics degree at cambridge. Pretty weak, right?
He then went on to do a Masters at UCL in astrophysics and got a first with distinction - he actually achieved the highest grades ever awarded in history for his degree course. He is now doing a PhD at UCL and is doing very well.
In all fairness, his work ethic massively changed once he went to UCL, but it does show how even very capable students can get poor degrees from oxbridge as the level of work and difficulty is so much greater than that at any other uni