Personally:
Chariots look like they have a lot of potential (at the very least, to draw attention away from elsewhere, but it gets D6 shots a turn and can put out a lot of hurt through combats too).
Goats seem great because you get that little bit of speed for objection-claiming and their special rule for charging means you may be able to avoid taking damage if you do somehow lose the fight (and if you win, getting two attacks with double strikes at S4 with the War Spear's +1 to wound is damned nice). However, they're pretty expensive points-wise and only D6 (so prime targets for shooting compared to your D7/8 main troops) - might be better to ally in some friendly cavalry, but these are pretty models and thematic and the Throne of Skulls rules can be kept if you keep pure.
Crossbows are basically a must. You can arm them with spears and use them as your second rank in a shieldwall if you fancy too. But the other Dwarfish armies have S3 shooting at 18" range, S2 shooting at 24" range, throwing axes or nothing (barring hte other ballista). Here, you have a chance for S4, 24" range and D6 (instead of the S2, 24" range, D5 rangers) - it keeps you in the running for a shoot-out with ease and can whittle the enemy as they approach you as well.
Ballista looks like it could be extremely useful too - not only does it do hefty damage to a target (as well as not scattering as far as most siege weapons) and not need line of sight itself to shoot, but it also has splash damage (can hurt models nearby the target too, disrupting their formations through fear or damage) and, additionally, prevents shooting across its own shot as well (and gets a free Heroic Shoot every turn anyway!) - it can be used offensively, defensively and for small amounts of area control. I'm sure there are other bonuses (incuding it's so pretty), but it's not a cheap unit and probably better for slightly larger games (although I imagine devastating in smaller ones - that being said, it makes it harder to reach objectives yourself due to having even fewer numbers than before).
Lastly, the mattocks (and this will apply to Goat Riders with them as well, although I'll add a note about them after too). I know this was your first question, but I wanted to comment on the others first, basically! Personally, I despise two-handed weapons. Unless you have Burly and/or Weapon Master (where you negate the -1 Duel Roll penalty), they are far too damaging to yourself to make use of the bonuses. The fact you have the choice of Piercing Strike or Bash for the special strikes is a nice bonus - I don't like to use Piercing Strike much any more though as Dwarfs are tough and, unless you win that fight (you're S4 anyway, so don't usually need the extra strength bonus), the Defence penalty is devastating; Bash is pretty situational and I'd almost always rather go for the wound/kill, but there are exceptions, like trying to knock a character from his steed or if you somehow outnumber something strong and want the rest of your troops to get double-strikes and force them to have to stand after if they survive. Additionally, by taking the mattocks, you lose your shield (no shielding option - almost always useful for the generally outnumbered Dwarfs - and lose a point of Defence, dropping to D6), which also means you lose access to the bonus for Shieldwall and can't help provide that bonus to other troops in the Shieldwalls (no D8 for this model and can't help other ones get to it either). As for on goat riders - Bash is pointless unless you receive the charge, in most cases. You get the change to knock them down from charging before the combat and if you win the combat after charging, knock most enemies down anyway - Bash then only becomes useful against non-infantry or if you didn't charge. Piercing Strike is arguably even more useless on them (again, unless charged) as you get +1 to Wound when charging from the War Spear - which equates to a roll of 2 on the Piercing Strike bonus strength (better than rolling a 1, same as rolling a 2 and 50% of the time, rolling a 3 is the same as rolling a 2 for this bonus strength wehn you look at rolling to Wound afterwards). Both of these negatives on top of the ones you already have (the point about shields and shieldwall less relevant here, obviously, but the rest still applies). Short version, really pretty, but in-game has SO many penalties, that I can't possible entertaint he idea of using them. Which makes me sad.
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