The new Ancient Ones are not, in my opinion, balanced. It is easier to beat most of them with larger teams than with smaller teams - considerably so.
Look at, for example, Quachil Uttaus. Assume, for the sake of argument, that each Investigator in the team is rolling on average X dice (regardless of how many Investigators are in the team). In a one player game, you'd roll X dice then get devoured. In a two player game, you'd both roll X dice then player 1 would be devoured then player 2 would roll X dice then be devoured; for a grand total of 3X dice when you're only looking for twice as many successes as in a one player game. In a three player game you'd be rolling 6X dice in total, looking for just three times as many successes. By the time you get to 8 players, it's 36X dice looking for just 8 times as many successes. The mechanics easily favour the larger teams.
Bokrug is another good example. It's far easier to buy off the Beings Of Ib if you have more players collecting trophies. You can collect gates and monsters far more easily.
I'm not necessarily saying this is good or bad, just pointing it out. The Ancient Ones in the base set were, at least in theory, balanced for teams of any size - or at least closer to being balanced. But I wouldn't recommend going up against most of the new Ancient Ones with anything less than 5 or 6 Investigators ...