Here you go fam:
Let's take a look at the admissions statistics. We'll go with Law since that's what I'm most familiar with. Now, King's College is (unsurprisingly) the most oversubscribed college. In 2016, it had 92 direct applicants (no open applications were allocated to King's). It gave out a mere 4 direct offers, and only 8 people who were pooled by King's were given offers from other colleges.
So if you applied to King's, you had a 4.3% chance of getting a direct offer from them, and a 13% chance of getting an offer from Cambridge at all (King's + any other colleges who want you).
Now look at Peterhouse. Peterhouse got 27 direct applicants in 2016 (also 0 open applications). They gave out 7 offers, and 2 people pooled by Peterhouse were given offers from other colleges.
So if you applied to Peterhouse, you had a 27% change of getting a direct offer, and a 29% chance of getting an offer from Cambridge at all (Peterhouse + any other colleges who want you).
The reason why the pool system isn't reliable is because being pooled doesn't guarantee you an offer, meaning you should never aim to be pooled and hope to get an offer through the pool. With King's you probably have about a 25% chance of being pooled, then another 25% chance of being given an offer from another college that has taken you from the pool.
That means that your chances of firstly being pooled by King's, then another college taking you from the pool, are 6%. Your chances of being given an offer through the pool are even lower if you apply to another college, since King's submit the most candidates to the pool of all the colleges. Hence why you shouldn't rely on the pool system to make your application chances equal regardless of the college you apply to.
You'll find a similar situation with any course (though not necessarily the same colleges), not just Law.
Source:
http://www.undergraduate.study.cam.ac.uk/apply/statistics (Select 'Specific year', identify a course, then select 'Group by College')