There was a big ruckus when he was revealed about how his ability should work as written, versus what was intended.
Basically people interpreted it as written as once you copy his abilities (via Quicksilver Elemental, for example), the "once per turn" clause goes away and you can do infinite combos super easy. However as was clarified here, I believe: https://twitter.com/EliShffrn/status/897894399329222656, the once per turn clause is tacked onto each ability of the caged cards. So copying Mairsil's abilities will also copy the "once per turn" clause.
So the legendary cards that are in the graveyard at the moment the ability that resolve gain that text... but legendary cards that enter the graveyard later don't... but those cards that did, if they leave the graveyard and eventually go back to grave, do they still have that text? Or did the zone change wipe that?
They're new objects when they change zones so they lose the ability. The ability is good for one play per legendary card, basically, you can't (just for example) activate the ability once and then continually replay and sac Saffi Eriksdotter the whole turn.
There was a big ruckus when he was revealed about how his ability should work as written, versus what was intended.
Basically people interpreted it as written as once you copy his abilities (via Quicksilver Elemental, for example), the "once per turn" clause goes away and you can do infinite combos super easy. However as was clarified here, I believe: https://twitter.com/EliShffrn/status/897894399329222656, the once per turn clause is tacked onto each ability of the caged cards. So copying Mairsil's abilities will also copy the "once per turn" clause.
It's like they learned from their Mairsil, the Pretender ambiguous wording.