Harder does not necessarily means "better", nor does easier. IB is tough time wise. Observation suggests the same applies French Bac. The final two years of school in England (and Wales) seem uniquely specialised when compared with the systems elsewhere in America or Europe. Our University system is naturally adapted to this and many degrees will assume that prospective students will have concentrated on just their main subject and a couple of supporting/complementary subjects before arriving. Obviously there will be some subjects where maining a greater breadth will be an advantage, but many which assume this greater focus.
The advantage is that a University degree here is often done and dusted within three years, despite UK Universities more than holding their own in international rankings. Plus the ability to select students who have already specialised can be expected to reduce the drop out rate, with students clearer about the reasons they want to study a subject and demonstrating a level of aptitude. The disadvantage of having a system which is different from our European and North American peers is that movement between systems, either inwards or outwards, can be difficult. Hence the appeal of IB for some international London parents. And presumably one reason why selective London private schools routinely have their students taking four A levels. American students will often keep up both maths and English to A2 not just to help with entrance tests but because they need to continue with both in the early stages of a typical Liberal Arts Degree. Many Europeans obviously take their mother tongue as an early additional A level, so can well end up with five.
FWIW DD is keeping five subjects up to A2 as she wants to retain the option of applying to Irish Universities next year should she get nowhere with UCAS this, which would mean achieving four absolutely top grades, plus it gives her the option of completely changing track should she need to. DS took five as well, useful as his offer was based on four A levels and he had an off two years day on one. (Five A levels seems mainly a science thing. I don't think you would get away with it and still have lots of time for computer gaming with humanities, though an EPQ on top of four humanities is reasonably common in London days schools.)
Generalist or specialist? Both mine were very relieved to get to A level and to be able to drop English and focus on subjects they found easier. There certainly is an argument for saying that British students, and my DC in particular, should keep up English or another essay subject to the end of school and beyond. Ditto for those who find maths hard. Our system probably does land us with too many geeky illiterates or the reverse.
Whether IB or A levels is a better preparation for a degree in England/Wales probably depends on the subject. If you are aiming for, say, Liberal Arts at UCL, then IB is probably going to be helpful. Engineering at Imperial, then A levels will allow you to narrow down your studies to relevent subjects from the age of 16, meaning you hit the ground running. We had a brief look at European alternatives when DS was applying for Economics and at Undergraduate level the UK seemed to have by far and away the best offer. (Shorter course, better ranked, more relevent content, wider employment/post graduate prospects.) And indeed the traffic seems to be overwhelmingly from elsewhere in Europe towards British courses. Universities have lots of experience, and top ones are very focussed on their international rankings so will want to recruit the best students. If there is a perception that they prefer A levels as a preparation, there is probably a reason.
Having worked in an international setting I can agree with Bobo, that there are cultural differences in an approach to analysis. It would not be unusual for Anglo-Saxons to look confused as a Frenchman launched into a lengthy, seemingly philisophical, argument about something quite straight-forward. Le Monde is hard to follow, not so much because of the language, though that is convoluted enough, but the argument. Similar English papers are much easier. (And third country nationals would say that American English, bar the jargon, was easier than Bristish English. We speak faster and use more vocabularly.)