(Im looking at you, Characteristic Defining Abilities.) The mechanic played a lot better than expected, especially with this years Magic mostly being played on Arena , which handled all the details of the mechanic for players
Being able to quest with Merlin and then challenge larger threats with your supported character makes for a strong turn, especially when enabled by a common-rarity card
Its an inconsequential card at Commander tables
Karrthus is a decent example of a high-powered legend that immediately pressures life totals, but there are cheaper examples like Maarika, Brutal GladiatorMaarika, Brutal Gladiator, Kresh the BloodbraidedKresh the Bloodbraided (who starts small but can scale absurdly in the right deck), and Coram, the Undertaker (who is on paper a little fiddly but is easy enough in the format to make gigantic)
One creature providing card advantage, ramp, and lifegain for stabilization makes for a horrifying threat, especially since it wont stay dead
As long as its reliably a 3/3 or 4/3 with vigilance, I think its definitely worth playing