This is how I see Buncles as working:
Buncles set up a strong defensive line early on between an early lvl 3 buncle (usually Jade buncle), Amethyst Eater, and Ruby Eater. If the opponent's units have range 1, Amethyst and Ruby Eater forces him to play multiple units to take down the frontline since the eaters heal + boost DF. If the opponent drops a high def tank, drop an Amethyst Buncle. Once that gets comfortably setup, Buncles sets up ATK reduction with Diamond Carbuncle. Finally, drop Agate Carbuncle for more agi and damage.
Also remember that a large amount of carbuncle files run Chamail/2nd Shrine Knight in the 3rd and 4th SS, so that provides a field wipe to take advantage of your opponent's field. This SS combo takes advantage of rush files having to place additional units by taking them down as well as boosting the carbuncles. For the first SSs, some people will run SP gen, some people will run SP denial. SP denial buys buncles time to setup, but can be a problem if the opponent also runs SP denial. SP gen is the safer option, but doesn't slow the opponent down.
So that said, how do you beat it?
-Pinpoint the support units/Return their supports and kill their frontline.
-Field wipes (Burning Sun/Resource SS, Allind, Zu/Rutina/Overdose, etc.)
-Attacking their Chamail/2nd Shrine at an inopportune time for them (ie when their field is closed).
-Bigs in general (since Buncles generally don't run returns/assassin skills).
I do think Buncles are a bit OP though, but I see it more as a problem with individual units rather than the tribe itself. Right now their ability to shut down direct combat seems a bit high, so I'd like to see that reduced.