No issue of running a BB with all loose balls provided you don't overload it with one extra. (you'll know right away because it won't adjust). The main purpose of retainers on bottom bracket and headset bearings is as a convenience in assembly. Imagine if they had to hand count and pack these in production volume on an assembly.

BTW- retainers normally don't undergo any strain in a bearing, so if you're having issues with them you're doing something wrong. Either you're putting them in backward which is very common, or you're not properly adjusting the bearing to extablish 100% contact with slight preload. When there's play in a bearing the loaded spindle presses down and the balls try to migrate toward the top which can tear a retainer apart. Even without a retainer the migration is a problem because it causes excess ball on ball friction under load, increasing wear.

So no retainer is fine, but be sure to get the adjustment right.