In the background, the Ammitara are painted as part of the Order of the Blind. A secretive and shrouded sect within the legion that was responsible for assassination protocols in tandem with scouting duties ordinarily associated with reconnaissance squads, they were a force that was often doubted to exist, even within their host legion.
The come with sniper rifles as their base armament and gain the stealth, move through cover, infiltrate and scout special rules that sets them up very nicely at the start of the game. Shroud bombs continue this theme very nicely.
What really sets them apart is their dedicated psychic power. As a brotherhood of level 1 psykers, they do not generate their powers as normal, but instead take a single dedicated power call Mind Killer. This is a blessing for 1 warp charge that allows re rolling of to hit and to wound, plus ignore cover. When it successfully goes off (and it should go off!) this makes the squad incredibly powerful on the board for long range sniping an assassination.
The scout armour is the most obvious weakness here, in place of regular power armour. But given they should be in hard cover sniping from range, that's more than okay.
The main problem here I think is whether the upgrades should be taken. The melta gun and plasma gun have fundamentally different roles to the sniper rifle, let alone ranges on the battle field. I think I would favour a plasma gun at 24 inch range, but its hard to want to take a melta gun here unless one can reliably get close to an enemy through infiltrating (etc.).
The Storm Eagle as a dedicated transport is certainly a nice option here though. One can always take it and then infiltrate the squad anyway, setting up the eagle separately.
A trio of builds to consider here.
5 Ammitara, Ammitara Fate with melta bombs and nuncio vox (150 points).
This is what I think would be a flexible and baseline unit to have on the battlefield.
5 Ammitara, 1 Plasma Gun (145 points).
An alternative baseline unit for infiltration purposes. The plasma gun can certainly benefit from the Mind Killer power here and be a real threat for a good points cost.
10 Ammitara, 2 Plasma Guns, Ammitara Fate with melta bombs and nuncio vox (270 points).
Take with a Storm Eagle. You have the option to set up inside the Eagle, or to go it alone with infiltration and be maximally flexible here.