As you say a Snowmobile would only be useable on one of the environments (at least properly ). From my point of view I would rather put the effort required for a new vehicle on something that can see more use (even though it will ofc have its own niche).
Shooting while jump packing is a limitation of the fact that we have no way to steer your aiming in three dimensions. No so much the game engines fault as the fact that we never made a system for it. Grenades and Stratagem balls would theoretically be possible as they would just fall with physics, but I’m unsure what the ramifications would be and it would be quite a big operation on the character movement states.