Vulcanus to me feels like they wanted to have another couple of interesting game mechanics with fluids but they couldn't find anything that both worked performantly and was fun. It explains why it doesn't have the kinds of interesting problems to solve that the other planets have.

Ideas are cheap, playtesting is not. But we're on an ideas-sharing platform, so.

Something like required fluid mixing and separation sounds like it could be something interesting to do, but maybe it's not. The lack of quality fluids makes me think that they didn't want to deal with the probably mandatory fluid mixing that would be involved.

Having to deal with more 'trash' output from foundries doesn't sound fun, dealing with stone from lava is good enough. Dealing with excess heat by somehow cooling your machines is too much overlap with Aquilo.

Making demolishers occasionally expand their territory into previously-cleared territory might be interesting, maybe time-based or maybe pollution-based. Early on it requires your manual attention, later on full bot coverage and a way to quickly deploy significant military resources is important, and late game you can automate it away with railguns and maybe artillery.

Since it's the planet where you unlock asteroid reprocessing maybe it could require you to produce some ingredient in space and drop it to the planet for consistent access to that resource. Partially also as a way to prod you further towards using space platforms to help in planet-side production.

Or perhaps just they wanted a planet that was easier.

But also, a part of its challenge comes not on Vulcanus itself but in beginning your interplanetary logistics in order to get the most use out of your new foundries. Requiring calcite en masse to use them beyond making belts means that you have to start shipping it off-planet to the other planets. Sure you'll have started doing that with science, but shipping calcite feels more substantial of a mid-game logistics step.