The only mega thread I think I ever liked and interacted with was Bloons TD 6. But that was also updated to a new thread each day so it didn't become a stale no man's land where comments went to die. (It was for submission of strategy/solution to daily challenges in the game. Some light competition in it too for most unique or efficient set ups.)

Other uses, but infrequent, were around major sales holidays. Subs like headphones or audiophile would make a mega thread for all the sales going on.

But I always felt the vast majority of them just weren't managed or interacted with enough to make them of use and often I'd see people say things like "you should ask that in the mega thread" and then the reply of "I did and no one replied" type stuff.