Fast Attack... such a crowded slot filled with lovely things I always want more of: Pathfinders, Tactical Drones, Vespid, and now Y'vahra!
That flamer profile is amazingly all-purpose. though I really wish they were 10" range or more, or Reserves worked differently (and/or clarified by FW later on to allow movement).
Seriously, the non-nova version does an average of 1.55 unsaved damage to an Imperial Knight, about 50% more than an astartes lascannon would, and the Nova's 2.33 is more than comparable to over 2 astartes Lascannons in expected damage.
And Imperial Knights are not really its expected target! Think in horror about what that would do to an exposed group of Tyranid Warriors, Necron Destroyers, Space Wolf Thunderwolves, and even our beloved Crisis Suits....
Support System wise, three choices stand out as particularly good:
ATS: King of 8th edition Support Systems. It makes non-invulnerable-save Vehicles melt even more against both main weapons, makes even the Flechettes semi-useful, and generally increases the shooting potential of an already extremely shooty unit (that's a good thing - a few points magnifying a lot of base firepower is good efficiency if the base firepower is fairly priced).
Target Lock: With an 18" Movement, an extremely close range Heavy dakka gun, and the fact that I generally want my Y'vahra to be able to operate somewhat independently (preferrably hopping from Tactical Drone wound bank to Tactical Drone wound bank, to keep that Y'vahra around past turn 1), I want to rely on as much base accuracy as possible when making use of all of that exceptional movement to get all that firepower where it needs to be. Relying on Markerlights for targets (especially more than 2) just doesn't seem reasonably reliable, so firing the IDC at normal BS is worth the points and worth the slot.
Stimulant Injector: With Drone Support, there's a basic principle worth following in almost all situations: if you've got a better save and can survive the hit reasonably well, take the mostly-worthless infantry shots and pass the multi-damage and/or high-penetration or mortal wounds onto the Drones. Stimulant Injectors on a 14w 2+ chassis with high mobility (i.e. can easily retreat to a nearby Drone Team for defensive support) means that you're making already weak infantry weapons even weaker, and can keep running at maximum movement (and thus maximum functionality).
Some still seem worth considering, but seem far more niche...
A Shield Generator (depending on the pricing) might be a worthwhile defensive slot, and allow for a bit more independence from Drone support (though still not a lot). Edit: How'd I miss the natural boost to 4+ when within 12" or in melee? Yeah, not worth the slot at that point, just go with Stims.
Early Warning Override: It has some potential, but other upgrades seem more on point. Still, if you Nova'd last turn, anything that deepstrikes within 12" is eating an average of 6 shots that mostly meet or exceed lascannons in stats but with roughly half the damage each, and anything that manages to break into the rare realm of Deepstriking within 9" of an enemy can end up severely punished. But again, mostly its got a single gun that would apply, and that just doesn't seem a worthwhile use of the slot.
Drone Controller: Drone Controllers are awesome, but I always look to see if something less obtrusive and with less opportunity cost in putting a DC on is around. Jumping from Tactical Drone ablative wound pool to ablative wound pool while laughing maniacally seems absolutely part and parcel with the Y'vahra's profile and presumed tactics, but a Drone Controller shouldn't be necessary so long as you can fit it onto other units.
And then, there's the the upgrade that's rarely useful. Multi-trackers can be so easily replicated (1 ML as opposed to 3), and have such an annoyingly restrictive requirement (shooting all weapons at the same target - not necessarily what you'll want to be doing with this weapon loadout), that even with their minuscule costs, I almost always find the slot better used by another upgrade.
But, can't ignore the elephant in the room: No more Assault Jump. That means the exceptionally close ranges of the Y'vahra's firepower (12", 8", and 6") mean that whatever is left of the enemy can and will be fully capable of assaulting you. You can certainly make assaulting from less than 8" away a very painful experience (7 to 10 6/-2/D3 auto-hits), but that's probably not going to be enough to deter dedicated assault units from turning your monstrous (and consequently monstrously priced) battlesuit into little torn bits of flesh and metal.
Really, that just means you need to annihilate utterly whatever you aim the Y'Vahra at (... often not impossible) and either have done so by isolating the target or by judicious firepower at the target and its allies from other forces of your own.
Call me cautiously optimistic.
I'm hopefully optimistic.
Yvahra deployed first on the field before using its Nova escape thrust and then remove it from play. So hes not considered Reinforcement like those Crisis who are kept in Reserve before game starts then arrive via Manta strike at the END of movement. Yvahra drop at the BEGINING of movement so yeah he can move.
If you have access to the leaked rulebook, look on the page "1. Movement Phase" (177, I think), particularly at the bottom paragraph of the far right column.
It discusses that the setup typically happens at the end of Movement, but that it can happen at other times as well. It them provides the universal rule that "Units that are set up in this manner cannot move or Advance further during the turn they arrive . . . but they can otherwise act normally (shoot, charge, etc.) for thte rest of their turn."
That is what prevents the Y'vahra from moving, and the Y'vahra seems to lack something that would grant it immunity from that reinforcements rule.