I think you can pretty much de-mystify the problem by considering energy-momentum conservation and the rest mass vs. relativistic mass.

A relativistic asteroid will have a much higher mass (and momentum) due to relativistic effects when seen from the near-static target than its restmass would usually be.

If the relativistic mass is much higher than that of the space station, it will smash through the space station, vaporize it and continue at the planet. The vaporized parts of the asteroid will keep the high relativistic mass, but expand from the core of the asteroid only at thermal speeds - which are unlikely to be more than a few km/s even in extreme cases - so over a scale of a few 100.000 km. the collision geometry will be in essence frozen, the impactor can't 'break up' in a meaningful way.

When it hits a planet and the relativistic mass is much lower than the planet mass, there will be a big crater, atmospheric devastation and possibly seismic activity, but such an impactor can be stopped.

When its relativistic mass is of the order of the planet's mass or higher, it will break up the planet / alter its orbit in a significant way.

The impactor will smash through and generate a linear fault zone through the planet around which immense energy is deposited and the planet will blow up around this line.

For the precise details of how the destruction happens, you need to run a model of the planet's deep geology, compute the stopping power of rock layers at various densities etc - but in a nutshell, the above is I believe what you can expect.