Other than small cosmetic differences the Century Hummingbird, Feda Dragonfly, Colco Thunderbird & Jabo are the same. All the parts are interchangeble between them. I haven't seen a Cupid so I can't say. I don't think the Aerohawk is the same. The Piccolo isn't a clone and judging by it's price it's of a higher build quality.
As for problems they all suffer from the usual ones. Mostly it's down to blade balance both main and tail & getting them setup correctly.
I think the worst problem with them is the appalling instructions that come with them. Lucky for us we've got this forum to give us all the info we'll ever need!!
Whatever you decide to go with you're the one who has to learn to fly it. Which is best is a question thats asked every week. In my opinion the best heli is the one you've been struggling with for ages that suddenly goes into a stable hover and actually obeys your commands. It's one of those never to be forgotten moments.