There are various ways to implement unipivot tonearms, so, at best, one can only offer generalizations about how they perform. A unipivot, by apply the whole force from the weight of the tonearm and cartridge on a tiny point, means that the bearing is very highly loaded and unlikely to rattle. But even though it is highly loaded, friction is quite low because the contact area is small.
From a practical point, most unipivot arms rock side to side when being handled, and that is disconcerting to some people. Whether it rocks and otherwise behaves in an untoward way when playing is a highly debated issue. Most unipivot arms employ some kind of damping fluid to damp undesirable vibration, but, even that practice has its detractors.
There are unipivot arms that go through quite a bit of trouble to reduce any tendency to rock sideways by employing stabilizing mechanisms. Graham uses magnets on the arm that are attracted to magnets on a free moving structure that is on the base of the arm to stabilize the arm. Basis unbalances the arm so that it has a tendency to want to roll in one direction, and that roll is then resisted by something that sticks out and contacts a roller bearing on the pillar of the arm. Of course, these added structures do increase friction and thereby reduce one of the purported advantages of unipivots and such structures will, like any added mechanism on an arm, become a potential source of mechanical vibration. Most unipivots rely on outrigger weights on both sides of the arm tube to add stability, sort of like the giant poles that tightrope walkers use for added balance.
As to sonic quality, one must judge that for oneself. There are a few such arms that have a lot of fans. Graham gets a lot of praise for their arms, as does Basis Audio. Although not as commonly seen, Moerch makes arms that, to me, sound quite good with the right cartridge.
