To echo what others have said - being farmed out for supervisions is quite common regardless of college, especially as you progress through years.
Libraries at mature colleges can be poor, this is true, however the subject libraries are must-visits anyway so it's hardly a major inconvenience.
A mature-college tour might puncture the jolly hockey-sticks and gothic buildings fantasy but frankly, pick your books up and go find a nice romantic corner somewhere in a different college.
I know people at all the big fancy colleges who have to live in and study in some very drab, modern buildings. You are attending the university, college talk is all rah-rah but life exists beyond those walls. Cambridge is what you make of it.
ps. the very term 'mature-college' conjures up images of 50+ mums and dads doing degrees, in actual fact you will find yourself to a great extent surrounded by 21 year-old affiliated students, post-army kids and Phd students, many of whom will be going through the dramatic academic tribulations faced at such a time and will make for very interesting company.