Point well noted... Palkia's supreme coverage and speed may make it the better candidate as its type coverage is superb. However, Palkia can be similarly taken advantage of by things like Wobbuffet, Lugia (no thunder), Sp.Def Groudon (soft check), and as you rightly said Grass Arceus (rain), Chansey, and Dialga; if the opposing team chooses Wobbuffet that Palkia is going to die if it stays on the field to kill something unless you predict marvellously well or crit with Outrage (surf in rain 2hkos, so there's that!).
Admittedly, Zekrom is easier to fiddle around with Bulldoze Ferrothorn and Ground Arceus being massive thorns in its side but, like Chansey, Arceus Grass, and Dialga are more common staples of a team and can run standard sets. Ground Arceus is a rarity because it's weak to pretty much every special attack, Bulldoze Ferrothorn will lack Power Whip or Gyro Ball and can be taken advantage of by things like Shaymin-S in the absence of the latter, and Sub CM Kyogre in the absence of the former.
Another interesting option as a Zekrom counter is Curse Ferrothorn but I'm just going somewhat off tangent now...
man i really wanna use curse ferro