Zombie template's a mixed bag, but part of that bag is a host of immunities, extra health, and natural armor -- they're quite substantially harder to kill. On the other hand, they're easy to hit (-2 def, -4 dex) and incredibly freaking slow -- half everything speed, and their natural movement speed cut to a third. They can be one of the harder things to kill for a starting character, but they're also almost always (barring ranged attacks, but that -4 dex and halved attack speed still mitigates that tremendously) a complete non-threat. You can just walk away and they can't really do anything about it. Similarly, reach weapons tend to be their infinite bane, since their reflex save gets kicked in the crotch by that dex penalty.
Basically, they hit really hard and they can be bloody hard to kill, but they're incredibly easy to completely trivialize. Not so much strong as annoying, ha.
Incidentally, you can 'l'ook and then e'x'amine (l->select critter-x) creatures to see... pretty much everything about them. Templates are completely detailed, most information, including spells (or at least spell access, in the case of stuff with caster levels), is provided. Incursion is very, very transparent in regards to most things.