It's hard to say, since I believe everyone got a Rank B so it's hard to really determine the "balance" in a sense til we see how A and S work.

Odds are it's possibly something like this:
B=Spawns often (potentially 30-h), low HP/easy
A=Spawns maybe 3-6hrs, medium HP/ full party difficulty
S=Spawns maybe 1-2 times a day, large HP, requires alliance to defeat.

So until we see how the other two difficulties rank and how often they spawn, can't really determine properly if B doesn't work as is, but I imagine since it does spawn rather often, that's the trade off for it's easy difficulty.