Loose balls are mainly better because there are more of them, so force is distributed over more surface area of both the balls (less force per ball) and the BB cup (more contact points, so less force per contact point).

There are some cases where retainers are usually desireable. Well, on case - headsets. They don't spin that much, already have lots of balls and contact points, and keeps the balls spaced-out properly.

But lower-end bikes often come with bearing retainers in the hubs and BB, neither of which is a good thing from a durability standpoint.