Monsters and Monstrous Cavalry for fun
K’daai Fireborn - Main issue with their Burning Bright is that since turn 2 you have to test their T.
Let me remind you how Characteristic Test looks like - If you’d roll less or equal to your characteristic, you had passed. In this case roll of “1” is an auto pass. Fireborn have T4, which gives them 33% to fail the test. If failed, you have D3 wounds for each failure, and with your W = 2 that means YOU JUST HAD FRAGGED YOURSELF.
Useless. Ignore. Don’t bother. Even if they had stats of a dragon and price of a infernal, they still would have to get into direct contact…
Well, ok, you may take some for lolz, as an expendable units, or if the battlefield is small and you expect CC since round 1 or 2.
The models look fab, have to say.
K’daai Destroyer - Starting with important stuff - Burning Bright with T6 - You pass the test if 1D6 = T or lower, so pass on 1-5 in this particular case (characteristic tests always fail on 6). Avoid all spells lowering T stats.
It does not have an official model, so it leaves you with bit of creativity or some of the alternatives. It goes on the base 100 x 150 mm so such as - Arachnarok Spider, Hellcannon, or 3 chariots one by one.
Like the Fireborn they are unbreakable but also unstable, which means that losing a combat will still hurt. Daemon in terms of spells from light lore, so watch yourself. And unit eater if you’d manage to get it in contact against block of foes.
I have one as it has highest I in the whole army (I5), and I needed something big and bulky to counter Stormfiends as soon as they’d appear (know your enemy). It’s awesome, but broken - things resistant to fire or magical attacks will stop it.
Had mine fighting against 40 Empire Halberds, Sigmarite Priest and 10 Great Swords. Killed most of them, then eventually destroyed them to a point they lost steadfast, made them run like a lil’ bitches, chased, killed the rest… only to be smashed by Hellblaster. All that with 1 wound left. Fair trade in terms of points, and epic LOLZ in terms of fun. Next time I will field LoA, I am sure my opponents will try to frag it to death ASAP, along with Iron Daemon 
It’s same thing with Destroyer as with Empire Steam Tank - first wound on it will hurt the most, and the rest of them is inevitable.
Bull Centaurs - solid choice. Taken only as a blocker to other Monstrous Cavalry or Cavalry. I had them facing Demigryphs, Skullcrushers and Dragon Ogres, so Great Weapon is the logical option. However with low initiative, in the battle on attrition against the aformentioned, they will eventually fail (and take few enemies with them). Maybe they would have more sense in 7th, but not in 8th edition that elevates infantry, war-machines and supportive magic rules - better buy more Dawi Zharr
Siege Giant - I have no good opinion about giants at all. Once my Chaos Giant had been eaten alive by giant rats as they wounded him more than he did them - He failed on LD, run away few inches, and they just followed finishing him off. This variant, even if armoured, still has random attacks. And that makes giants in my eyes un-re-li-ab-le. Period. If you really want to spent 200-ish points on something, better invest in cannons.