Yoshi also lacks Mythra's overall mobility and doesn't have access to Pyra. Comparing the moves in a vacuum doesn't always give you the full picture.Ironic, considering that when you compare Mythra's f-smash to Yoshi's f-smash, both attacks hit on frame 14, but the latter deals more damage and knockback overall. And of course, Yoshi's head is intangible during the attack. But when you think it over, Mythra's f-smash does have more range, so I guess being less powerful makes some sense.
In terms of Lightning Buster, it does need to be fully charged to gain the best possible KO potency. Not many players are going to let Mythra pull that off.