Here's my take on it. Here's the original box art:

It's got Jyn, Fenn, Diala, stormtroopers, and Vader. Basically, a relatively prototypical IA mission, and includes all figures that can be found in the Core box (Han, for instance, isn't on it).

Twin Shadows infamously features Boba Fett. Yeah, he's not in the box. Little is, really. Really, the only minis they could show on it are Biv and Saska, Heavy Troopers, or Tusken Raiders. The Heroes are hardly iconic to show on their own, the troopers aren't really that different to differentiate between the core box. I guess best case scenario, the box art could've featured Biv and Saska fighting Tusken Raiders, but they (understandably) decided to feature the main draw of the wave instead- Boba Fett.

Wampas and Verena are in the box. Diala is in core, but you definitely need to own Core to play this, so that's fine.
Echo Base troopers in the background (I literally just realized for the first time they were even there) don't make the cut in the box as figures, but they may or may not be heavily featured in campaign regardless.

Like Twin Shadows, you've got the conundrum where the only figures in the box are the heroes and relatively uniconic draws big enough for the sale alone (ugs, Wing Guards). Basically, the same reasoning for TS.

Jabba isn't in the box as a figure, but it's Jabba's Realm. How could the cover not feature him?
By the way, Salacious Crumb and Bib were never made into figures, and I'm pretty Oola never shows up at all. That Jawa in the background wouldn't come until a few months later, and I'm still waiting on a Quarren.
But I've never heard people complain about that.

As far as figures go, probably the most simplistic cover. Ko-Tun is in the box. Jyn is in Core. Easy peasy.

Death Troopers are in the box. Thrawn is part of the wave. At this point, I see it has a lot of the same issues as the other two small boxes, in that it relies on its non-included as a figure villain for icon. But unlike the other two, it also features a unit in the box, in Death Troopers. So, as I see it, it's actually a step in the right direction, even for someone who is against showcasing non-included figures on the box.
But what it really comes down to, I think, is that this art is more than just for the box, but for the wave itself. If you're playing with the wave you don't need to buy the corresponding figure packs, but that doesn't stop the characters from still being a part of the game in some form, and even within the game's narrative.