>>1422775Shadow style rear mechs are a single pivot design - essentially, only the cage is allowed to rotate. Older shimano style were dual pivot design ( pic related, marked with a friendly red circle ). This other pivot was counter tensioned to the the cage one.
Dual pivot rear derailleurs track the cassette better then single pivots and thus are more tolerant of different cassette sizes. Shadows are, literally, bound to a single cassette size and if you sway too much from it, then shifting quality degrades _fast_.
Shadows are less flexible, so they hold the chain a little better, but if you sway too much from their 'designated' cassette size then they literally stop working.
