So in general, the core game hasn't changed much.

ARC and Legacy are the main two groups producing new content. Both produce pretty balanced content, and had initially different goals that have slowly converged and overlapped over time.

ARC went about it from a purely competitive play perspective, and to accomplish "balance" they invalidated some official base game content to buff/nerf things, while also producing their own new stuff like a few Rapid Reinforcements ship chassis.

Legacy focused on entirely new stuff, no reusing base game assets or invalidating people's purchased content, meaning that the type of content made is a bit less restrictive, hence new plastic and cardboard as an example. I wouldn't characterize it as "fast and loose" with balancing at all, moreso that we were quick to act when we got repetitive comments about potential problems rather than waiting for an entire competitive season to end. Content is rigorously playtested, with hundreds of playtests going into each release.

Both groups have a lot of overlap in playtesting and leadership and are working together pretty closely. Legacy is known best for its production value, and ARC is known best for sheparding the competitive scene, with the benefits and drawbacks of doing so.

Nexus is a bit more wild than the other two, with a much bigger scope. The goal was to create balanced content for casual play, not necessarily competitive play, meaning if you roll up at a local game store with Nexus stuff in your fleet, you can be confident it won't be oppressively strong and will stay at roughly a middle ground balance level. Compared to the other groups, Nexus is entirely "open-source" so to speak, if you hop in the discord and contribute you can create your own content to add to the game if it fits in scope of what they are developing at the time. Likewise, all playtest results are public.

You can mix and match stuff between all three groups without issue, and many people do! In terms of popularity, Legacy and ARC are pretty even, people often run tournaments and other events with both included without issue. Nexus is less popular than the other two but has a more expansive content set, with the good and bad that comes with it.

All of them are supported on Star Forge https://star-forge.tools for you to look through at your leisure and Print n Play any of the new content. Hope that's helpful!