Complex sentence: The advantage of a Grothendieck universe is that it is actually a "set", and never a proper class; the disadvantage is that if one tries hard enough, one can leave a Grothendieck universe.
Split into simpler sentences: The advantage of a Grothendieck universe is that it is actually a "set", and never a proper class. The disadvantage is that if one tries hard enough, one can leave a Grothendieck universe.