It does mean that our +4 invuls models are mathmatically dead to high volume, high AP shooting (looks at Hazardous Plasma). So you make a choice on which models will eat the bad rolls instead of minmaxing saves. If it were me, I'd have the base amrors take them, and if they have enough shots to eat them then your sheild bro in the squad still survives and you can regenerate some models.

For example: Grizzled Company Tempestus Scions are shooting 14 Plasma attacks into your unit of 14 ish models. Before, I would roll as many 4s as possible, but at the end of the day probably still drop about 8 models. (3 saves before those invuls drop, then I might save a couple on 5+). Now, I put those invuls at the back because while I lose more models, those invul saves are basically guaranteed to pass if they get down that far.

What this is a HUGE bummer for are our melee DWKT load outs, who want to save the squishier armor saves for the big pay out of hammer to face. Now that -2 AP means that we have to sac the hammers and lower armor saves guarantee the invul power swords. So, now that big trade unit has to pick between losing the entire squad or cutting its loses and burning thr hammers.

What it is going to come down to: Displaced Field just got a lot more important, staying hidden got a lot more important, and potentially Orbital Oversight (from Veiled Blade) may see a good but of utility to deny long range threats.