I disagree. I have four of each in my Mega-grow decks.
They just need the right support cards and proper timing.
You are right that they are not bad cards but bad cards for legendary in my opinion. Most cards are good with right support and timing.
Granted, they are legendary, but they don't have to be OP.
They are legendary because their design is unique and they have abilities that can get out of control if played correctly. They are great cards, but they aren't that easy to play well, specially Double Mint.
Also, many competitive decks don't include legendaries because they are too complex to handle.
