IMO, steel reels have two advantages: they take less chemical to cover, so for 35 mm can use a little less developer, and they can be loaded as easily when reel and/or film are damp or wet as when bone dry. Plastic reels generally load best when both film and reel are bone dry -- or else under water (but may scratch the film edges if the emulsion is wet), but you can load two 120 rolls onto an adjustable plastic reel, one after the other, as long as you have a way to keep them from overlapping; 120 size steel reels use thicker guide wire, so can only take a single roll unless you master the questionable technique of loading two rolls base to base (which seems likely to guarantee having to refix or rewash to remove dyes from the base side).
I wouldn't even try to load steel reels in a bag -- but on a counter top, they're quite easy (at least in 35mm), as long as they're not bent.