Can someone tell me a bit of strategies to use with expansion?

Human and undead units in expansion seem overpowered….

and elves seem to be really a bad choice.

Can anyone shed some light as to the use of the new units?

Thank you.

I have the same feeling about the races. It's not impossible to win with other races, but it's harder. My solution - pick the two strong races if they are available, don't pick elves if you not the last player - starting area with good food sources is too important for them. I wish I could choose races freely, but they are not that balanced, so I almost always choose them in the same order (Humans>Undeads>Chaos>Elves). 

I think personal taste and playstyle is a factor here. To me, Chaos is strongest in the expansion (the berzerker upgrade is so potent!). Undead seemed strongest in the core game, but they got the smallest boost into the expansion.  For instance, the Frost Wurm is awesome but it's not heads and shoulders over the Death Knight.  The Vampire replaces the Necromancer.  Since the Necro is the faction's best unit (in my opinion), the Vampire is not really useful unlesss you've already deployed all the Necros.  The upgrade cards are where the Undead really fall behind.  A bunch aren't that useful (swaying neutrals to your side, vampire upgrades), though they do get very nice archer and necro upgrades.  

I think the elves got some very nice upgrades in the expansion.  The Treeman is weak for a hexagon, but it's very nice just to have a beefy unit to put damage on.  Cat riders are sometimes better than warriors.  A bunch of their upgrades are superb.  And they get the advantage of tie breaking favortism.

Overall, I think the factions in the expansion are nicely balanced.  You just need to play to faction strengths.