With an intelligence that some within the Mechanicum consider to be on the borderline of an abomination of animated silicon wafers, the Scyllax are written to be highly durable, intelligent with a full battlefield role. That said, they are comparatively rare and typically serve as bodyguards of the high ranking Domina.
With 2 wounds each and toughness of 5, these machines are tough and relatively worth their points in the stat line department alone.
The have access to fearless so long as they're within range of a cortex controller (or suitable Iron Warrior with the battle smith special rule).
Their close combat array is interesting as it can generate extra hits on wound rolls of 6, or be substituted for an enhanced strength and AP singular hit which is well worth while if they're going to up against terminators or terminator equivalents.
As if these weren't good enough, they get a bolter with a better AP and longer range, coupled with a rad furnace engine that gives the usual radiation poisoning penalty to opponents in melee. Blimey!
I would have preferred them to be a lot faster frankly. That way they could have been superlatively deadly. I think they're going to need some kind of transport and they probably want to be with a high ranking Dominus.
If they do happen to be outside the nominal range from a cortex controller, then they don't move either. That could be bad. Particularly with average Ld.
16 Scyallax, frag grenades (565 points).
A large squad for killing lots of things in combat or with their bolters. Take a Triaros for transport to taste? Relentless with rapid fire is very nice here regardless.
10 Scyllax, frag grenades, Volkite Chargers (445 points).
With volkite, we need to have lots of them equipped with them to make the most of it.
5 Scyllax, frag grenades, 1 graviton gun (195 points).
Cheaper than terminators are possibly a bit better assuming you have a cortex controller somewhere nearby and you can get in close combat to use their high S hit against T=3 terminators (reduced thanks to all that radiation) -- instant killing as well in all likelihood. I like this arrangement, but feel free to swap out the graviton gun for a plasma gun or a melta gun to taste.