How difficult is it to break into IB?

Disclaimer: I'm a summer intern, so can't speak on FT. I'd say it completely depends on your situation. A top target student with a 4.0 won't have much issue breaking into IB as long as they're reasonably competent and vice versa. All you can really do is set yourself up the best you can to maximize your odds of securing offers. With relevant experience, a good school, a high gpa, and networking, you'll likely land something-now, how good that "something" is depends on a lot of factors, luck being one of them. Obviously, none of those are required to break in, they'll just make your life easier. That's my 2 cents.

Depends on how hard you prep and on your background like others mentioned. Also depends on what kind of firm you want to go to. Generally speaking, competition is intense.

Bulge brackets and boutiques like LAZ/CVP/EVR have around 2-4% accept rates (guesstimate). In my school conference composed of mostly semitargets, ~25% of applicants got a first round at one of those boutiques and only 2% got internship offers.

Even at the private equity firm I'm interning at now, the acceptance rate was 1% out of a bit under a thousand applicants. When even those stepping stone internships have such low accept rates, it's easy to see the difficulty of breaking in.

Full-time recruiting is another matter and is even more difficult in general.

It depends largely on your individual situation - when I first learned about the process for recruiting and the "target" and "non-target" differences, I was worried that I (having gone to a non-target school) would have no chance of breaking in. However, I've learned that it is actually far more nuanced - different schools have very different prospects for IB, and even with regards to geography, a non-target school closer to the city you're recruiting in, obviously, has a better draw than one father away. I was lucky enough to attend college fairly close to the city I was recruiting in, so it made getting my foot in the door slightly easier. 

Besides this differentiation, the only way to reduce your disadvantage (i.e., at least get a first round) is through networking - I networked my ass off and entered a dozen processes at major banks (MMs & EBs) and industry boutiques, superdayed at ~6, and converted several offers. Though counterfactual, I think it makes sense (in my position) that without networking I probably would not have had even a first round at most of these shops, because our school is so underrepresented. 

I would also say that the technical portion of interviews vary by a wide margin, and that some banks place a higher importance on cultural fit rather that technical acumen, or vice versa. If I could've done things differently I would've been more precise with my networking efforts (not overdoing it) and figuring out more specifically what banks look for, and tailoring prep more closely to match.