By the numbers, Halfling is the best attacker in the game. So what is the property of this magical "core" that weakens halfers?
Only Orc seems to compete to my mind, due to the +gains... so I guess you could use a few halfer warrior "breakers" in an orc core... but bleh, that sci malus is brutal, I'd hate having to work around it. Plus, more halfers means you don't need dedicated "breaker" type attackers - you can always just send extra elites situationally instead.
UD is just a compromise between orc and halfer - power right in the middle, no +gains, randomly gets a massive buff when plague procs. (Er, well, gives the next guys a big buff.) No thief potential though, so you need fewer attackers to make room for more T/M or A/Ts or something.