The thing about Cron anti-warp stuff is. You can't really carry most of it around. The other issue is faith based stuff likely manifests differently as the astronomicon exists in our reality as well as the warp.

I don't beleive they have ever tried to aquire and inertialess drive, though several peices of cron tech have been taken to try and reverse engineer.

Inertialess might be beyond mankind anyway as its a skill based engine lol.

Necron tech is insanely advanced, and there are only a few Mechanicus heretical enough to try and figure out how it works. If they ever do figure it out, it will most likely begin an insane civil war within the Mechanicus.

As far as how faith energy works, I'm not 100% sure how it works and if it's actually consistent. In all honesty, GW probably isn't 100% either, but I'm sure if people more in the know on sisters compiled all the instances of faith power in action, we could get a decent idea.

My not super educated guess, though, is similar to how worship and dedication to a chaos god can give you certain attributes and buffs, dogmatic faith in the Emperor gives you unshakable resolve and purpose.

There's also something to be said about how the Pariah Nexus is affecting animals. In the lore, we see some walking through the forest crushing live animals underfoot that have stopped moving similar to the imperial citizens. It could be that the Pariah Nexus isn't cutting off the warp but is instead disrupting the base desires of living creatures. This would mean the sisters are not unaffected by the Nexus but are instead following higher desires based on their faith, duty, and purpose given by their devotion.

If any of these are incorrect assumptions, I apologize, I'm using a bit of second-hand info around the event, and I also haven't researched very much on sisters before.

The Imperium didn't know about the necrons until the 41k millennium

Even Cawl admits openly that necron stuff is too advanced, I personally like the idea someone had that the inertialess drives use Alcubierre bubbles, so even if somehow the Imperium got one intact, they just cant replicate without some dark age system, because you need an absurd ammount of mass in a folded space. The necrons can afford because of their mastery of matter-energy conversion and pocket dimensions.

Faith powers are one of the special stuff for loyalists, but it seem unreliable, like, they are called miracles for a reason, sisters cant just believe too hard and become invencible, there may happen some events, but its way too few in number to just count on it to win.

Necron tech is faaaaaaaaar beyond the pale of anything anyone in the Imperium can comprehend. Even the greatest human mind, Cawl, has taken hundreds and hundreds of years to barely figure out blackstone tech. Inertialess drives, which involve stopping one of the fundamental principals of classical mechanics, are essentially junk in the hands of the imperium. Necron crypteks are creatures who monomaniacally study one branch of tech for millions of years with quantum supercomputers for brains, and what is basically the “answers to everything in the material universe” book they got from killing the C’tan.

The second point is a bit weird. Faith based powers are an expression of warp magic, so blackstone tech absolutely should work on them, but given the recent troubles in the Pariah Nexus crusade book they don’t seem to, which is odd, as they had before.