The full heal plus pinpointed damage at 0 cost is indeed very good and I'd like it to at least become random, but putting a unit in the 40 hp range is a significant nerf with all the 40 damage SS around.
The problem with synergy files is always that if you can prevent them from setting up they're generally weak, while if you can't prevent the setup and can't OHKO (multiple) units a turn you might aswell give up. If you can beat Ruby Eater in one turn consistently the buncle player may aswell give up, but if buncles can consistantly heal back up...
SS like chiruru and assasin are generally a good way to help fight buncles (for rushes), with 2x40 damage or gafc SS early game. Rasam is hardly ever a bad SS, and works well against them too (for a lot of files). Any card that can buff attack like kurina and annarose can also help, aswell as any unit that can do major damage with a single skill/attack. Falkow can just use return problem units while having units that can OHKO buncles (sea serpent, grim count, WK if sphere level is high enough, Urgrant,...).
It's currently a popular file that gives very annoying matches to files that don't have what it takes to take them down, but a lot of care should be taken such that the file doesn't lose out too much power against the files that are already good against it (or we'll get another file like the refess prebuild that people only buy to get lappiere). I think Command dolls are likely better than buncles as a synergy file and would get simelar complaints if more people had the cards to play it. Orks might also get those complaints if more people had them. Also CMD dolls should probably have easy wins against buncles (hard to tell for sure, as I can't test it).
As I haven't been playing recently I don't have data on whether they are really too strong at the moment or if they are balanced enough. If you want a decent chance against them without going for something expensive like CMD dolls, the sea serpent prebuild may be useful (shrine knight prebuild may also work with a proper SS lineup). If you have the proper rares you can even make files like augments have a good win rate against buncles, the problem is many people don't have acces to those cards, just like how many people don't have acces to CMD dolls.
"Truly, if there is evil in this world, it lies within the heart of mankind.'