One thing about the Erebus - an Assasin erebus is almost as powerful as a minion erebus. The other team has a crapton AOE, a TB and a mine reg or something, the erebus just goes pure assasin, and with batswarm/bite/mass charm, any TB or Reg that comes near an erebus alone is dead. Sedna and UB might be able to take it 1v1, but the amazing escape ability of an erebus makes up for a lot.
Sedna is simply essential in any good team. Sedna + UB/Ere/Oak vs any twosome without a sedna? The sedna side wins.
Frankly, the surprising statistic, to me, is that Reg does as well as it does. I've only ever played with one reg worth bothering with. Once a game comes down to flag locking portals, a reg becomes almost worthless, since it can't stand in a fight against anyone. Kiting isn't very helpful when all someone wants to do is lock a flag and leave. The complete and utter lack of stuns/interrupts does not help this at all. This is one of the main reasons you'll never see a fire TB or a reg in a serious premade game. Sure, you can get away with it as long as you have two tanks to hold lanes (for cataract) but it's a disadvantage. If you don't win the game before it comes down to portal locking at the end, the reg/fire tb side should lose.
Reg vs a UB with spit? it's not even funny. Reg vs Assasin erebus? That one actually is funny, because the assasin erebus ganks him all day long. The fact that a good team will have at least one of those? Not good for reg.