How are the gameplay improvements? What of veteran levels?
Some general impressions:
I can't comment on the new weapons/skills as I wasn't testing them over the weekend, but there's tons of new stuff for everyone - armors, weapons, food, drugs, new components for old blueprints, miscellanea from belts to grenades and entirely new gadgets. Lots of new ways to temporarily boost everything from skills to base abilities to various combat stats. Will take time to discover all the new stuff and assess the possibilities they offer... you're thinking of power creep? Me too, but so far the expansion has some of the hardest fights of Underrail. Noobs rushing to Black Sea will get rekt for sure.
One thing I like in particular is increased number of small details in the new areas. Not graphically (although there's that too), but there are plenty of points to observe in the environment when exploring on foot. Probably not a big deal for most players, but I just like that kind of extra interactivity. There are other kinds of new environmental interactivity as well, like fire burning overgrown plants to cinders. Most of the new areas are in Black Sea and the southern waterways, but there are a few small side areas in the south. Unlike the countless "somewhere in Lower Underrail" areas, all the major geographical features of the south waterways are named and referenced in dialogs, not just in the global map. A nice worldbuilding touch. They also feel really alive, there's plenty of traffic going through the main routes. For an enterprising murderhobo this means opportunities for piracy. Which brings me to...
The jetski system. It's pretty much as explained in the devlog, and it's great. Jet customization is full of meaningful tradeoffs between weight, speed, power drain, and stability. Jetski frames differ in number and size of part slots, base stability, hydrodynamicity (speed modifier depending on engine power and loaded weight), special abilities like turbos or built-in weapons, and defensive stats of durability, cover (chance of attacks hitting the jet instead of rider) and resistances. The jetski models range from speeders with gaudy flame paintjobs to lumbering AlFabet-boats that can easily move your weekly junk haul
and tow your
hard-earned stolen jetskis. Of course there are also heavily armored combat jets with rocket launchers or various energy weapons, if hand-held weapons don't fit your idea of naval combat.
As for the Black Sea campaign... nobody has gotten far yet, but it's more dynamic than anything you've seen in Underrail. That's about all I can say without spoilers.
Veteran levels are pretty much as you read from devlogs. People didn't like veteran feats, so that idea took a backseat to feat specializations - a whole new layer of character building, speccing into some aspect of your favorite feats to further improve them. That's the good stuff. Veteran feats are still there, but they're pretty niche - not a part of every character, unlike feat specializations. There's a bunch of new normal feats as well, and not just for the expansion weapons.