Mass Polymorph only ever saw much play in Jalira, Master Polymorphist EDH decks. Jalira is a fun Commander who can flip utility creatures into huge creatures with big effects for crazy big value. Unfortunately, her ability doesn't allow you to hit Legendary creatures and the critical mass of good targets for Jalira to hit hasn't happened yet.
The other deck that Mass Polymorph saw play in were some builds of Talrand, Sky Summoner in EDH. Talrand typically plays very few creatures in exchange for a bunch of cheap instant and sorcery spells in order to create a whole bunch of 2/2 flying Drake Tokens. If you were to have a Talrand build which actually ran a fair number of big bomb blue creatures, the payoff could be decent.
Does the Instant speed version of Mass Polymorph actually make that sort of Talrand build possible? Can a functionally better version of Mass Polymorph for the same 6 mana give Jalira some extra fuel to be a better Commander? The answer to both questions is yes.
Is Synthetic Destiny a relevant card in EDH? Beyond the right builds of Jalira or Talrand, not really. However, some builds of Ezuri, Claw of Progress have given this card a shot, and he is a highly popular Commander. This card is definitely a consideration for EDH decks that pump out a bunch of tokens and can provide some surprise value for a relatively modest investment of resources.