Double mint and Bananasaurus are bad legendary cards.

Bananasaurus Rex is one of the best cards ever created though.

Yes but Bananasaurus is not.

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. 

Both double mint and bananasauras rex are v good cards. Today my opponent played double mint on turn 2 and a i destroyed it easily. NEVER play double mint on turn 2( just like paparazi zombie & zookeper should not b played on turn 1 & 2), also be careful with zombie heroes who use rockets as they can easily destroy bananasauraus rex/double mint. yes double mint needs protection when played ( using torchwood or any teaaup plant). once my opponents doublemint got to 62/60.

Playing double mint on 3 is also fine, but if you play it at 4, you won't likely get much profit out from it since games usually end fast.

You play double mint in a way that it forces your opponent to address it. It is like forcing your opponent to spend brains on it instead of on your strong plants.