I think the problem with BG/x isn't that its not good, its when the decks start to feel like different flavors of the same <x> value deck, which is more or less the final resting place for a catagory of decks often built around general goodness and ubiquitous answers. I don't think its even really an issue of power level:
thoughtseize decks in their various forms have generally made up a (not surprisingly) disproportionate % of the standard metagame for about a year now.
If absolute power corrupts, generally good creatures and ubiquitous interaction corrupts a lot of drafters. When going into the unknowns of a diverse draft format, it can become a safety blanket strategy, where week after week they avoid the cube's metagame (you know, that thing you took month's to design) in favor of drafting generic goodness against whatever they face. Whether its
siege rhino or
thragtusk,
putrid leech or
rakshasa deathdealer,
languish or
damnation,
eternal witness or
den protector, those cards will never actively be bad. Facing removal heavy decks?
Profane command or
whip of erebos to disrupt their removal. And of course, they have access to the most ubiquitous of answers besides actual cheap counterspells in the form of cheap targeted discard.
In diverse formats thats a smart strategy, especially in cube where you probably lack impactful sideboard cards. But as someone running a casual play group, those sorts of value decks just get boring real fast, especially when you realize that players are just using it as crutch to avoid learning the meta you took so much time to craft. The only thing really keeping the deck in check is that they are unlikely to have multiple copies of discard as good as
thoughtseize.
Not that I'm bashing the color combination--I really love my
lim-dul's vault powered BUG decks. I'm just saying that I can understand some designer's frustration with wanting more personality in those decks. I think providing them with a variety of different engines is a good starting place. I like the idea of the
den protector deathmist raptor engine, for example, from standard (though doubt its directly portable to cube), and I've enjoyed vault and
insidious dreams in my BUG decks to setup big self-mill plays.
Genesis is another good engine card for the attritionay
tortured existence version of those decks.
I don't feel its an ideal aggro setup for a lot of cubes, but its possible with a little restructuring to green 1s and blacks 2s.