Background.
The dread silica anima are not dead. Known only to the Emperor and the Dark Angels, these artificial intelligences are not Mars-born, but left over from ancient Terra and born of malice with similar intent. They have been somewhat neutered by the Ironwing, but still lack restraint and could potentially turn on the Dark Angels as much as they wilfully eliminate their enemies. These are the Excindio: Dark Age AI nightmares.
Strengths.
Wow. This unit is in no small part rather incredible. The sheer fact of its existence in 30k is amazing in the first place, and its tabletop rules back it up. These automata are sheer death for most things short of special character models (and it'll happily kill many of them too). They will kill most things in close combat, and with the right weapon load out will do the same to vehicles at range as well. Honestly, they will get the job done.
Weaknesses.
There are several weaknesses to consider with these beasts. Firstly, they stand a chance of turning on your own army. Hence the best way to play around this is to keep them away (or keep them nearer to an enemy -- this is a must). Secondly, with upgrades, their price tag starts to inflate to a level where you might as well take the Lion instead (equally, these machines are heavy support choices, not Lords of War). Or maybe some Mechanicum allies for a better health and safety record? Regardless, they can be equipped with a kill switch should they go truly rogue, but I question the utility of this given the close range that a controller has to be within to use it.
Builds.
There are several ways to construct these machines. All deadly.
Excindio with 2 Atomantic Pulse Cannons and 2 hunter killer missiles (380 points).
A ranged option to bring down most vehicles in the game.
Excindio with 1 graviton flux projector, 2 irad cleansers (340 points).
Interesting close combat variant, but consider carefully how it will get in to combat?
Excindio with 1 Atomantic Pulse Cannon (330 points).
My favourite build here, as it provides the best of ranged and close combat ability for an optimal price.
Excindio with 1 Nerve Induction Shredder, 2 plasma repeaters (340 points).
Something to take care of hordes?