It's not necessary to meet a monoclan to have a normal bonus. You forgot that in a fight you can meet 1-2-3-4 characters of the same clan (biclan system), it depends on the random. In whole it's a risk but -16 attack is worth of it.
I do not think the bonus is going to be Brawl, but if it was, it wouldn't be bad. Most new clans came with bonuses to counter other specific clans, as Raptors did With the Stop Atk Modif, so having a bonus that especifically counters Mono Clans is not so farfetch. They could directly target clans such as Rescue or Roots, who usually play Mono. Even if most of the time their bonus would be -7 or -8 atk, it'd still be good, Sentinel and Junks increase their attack by 8 and Sakrohm reduces it by 8 as well, so... Not really a weak bonus if you consider it could go higher against mono clans.