Ruinous: It is quite an advantage to get all three of the orc buildings up and running quickly though and the orc are decent cheap fodder, however Centaurs are probably the best alliance to aim for given how good those units are. Sadly I'm stuck with Orcs now from the sounds of things, but it isn't a disaster ;)
Well the Orcs themselves are decent units, but their other two buildings were pretty weak IIRC? But no hardly a disaster :) As for what alliance is best - it's hard to say. Centaurs are good, but killing Ogres can be a pain sometimes. Elves are amazing, but finding those kidnapped maidens can be an even bigger PITA :)
I've actually started to prefer Lizardmen. They - like Orcs - are pretty decent units (actually better I think), and you can usually find them very early on. And there tend to be a goodly amount of their provinces around to take over for free as well.
But it really varies. Also I guess the permanent Orc alliance has the main benefit of getting to take any Orc province for free right out of the gate. That can be a pretty big bonus, right there. Good stuff to do in turns where your units are healing.