I did the IB myself, although a long time ago (1980). I went to an international sixth form college that did only the IB, so I have experience of it only at that level.
At that level, it gives you a good combination of breadth (six subjects, plus Theory of Knowledge) and depth (three highers and three subsidiary subjects. It is more demanding than UK A-levels and I suspect that it's not for the academically shaky. Having said that, almost everyone in my year passed the diploma. It was a very good school, however.
Another positive aspect of the IB is that the curriculum avoids the narrow focus on British history, literature etc that, at least when I was at school, characterised A-levels. In the Higher Level History course, I specialised in European History, but other students specialised in Asian or African History. Within the European History course, we also spent time looking at Wars in the C20th across the world (e.g. Vietnam) and at World Development issues. Similarly, the English as first language Higher Level syllabus included world literature in translation and we looked at Scandinavian and Indonesian authors. I found all this very refreshing after the narrowness of O Levels. Incidentally, DH teaches at Oxbridge and finds that students who have done the IB are well prepared and well rounded academically.
On the question of what happens if you are not strong in certain areas - I had little confidence in maths, but I still did reasonably well. Students can do maths at very basic, basic or advanced levels and this caters well for the spread of ability.