Why are mythic seals so rare if they're so bad?
They just needed 1 day of testing with the mythic seals to see 5 set bonuses are way too op to make the 2/3 unique ones good, and the one that reduces set charms needed for the bonus is bad if its going to roll some garbage affix anyway so a 6 slot legendary seal with 25% crit dmg and a usable set affix are just strictly better. Even if it has a dead set affix its still better than a somehow 100 hour plus drop for a SINGLE mythic seal that HAS to drop as the set bonus one to even consider being usable if it also rolled a good affix too.
At the very very least it should be like actual mythics and have static affixes. At worst it should roll something like 20% all dmg. Either that or if they want to keep it in the current trash state they should be literally 10 times more common so you see one every 10-20 hours you're grinding in t12 minimum, not once every 100 hours if you're lucky.