excluding attacks from within I think planets are safer from space strikes from other civilizations than a space installation, planets as a whole can be rigged up with a missile grid, and possibly be rnendered near imoenatrable with future technology. With enough engine power a planet might even be rendered mobile
For a spacefaring civilisation, planets are sitting ducks.
You can rain down asteroids on them, you can do all sorts of nasty things to the atmosphere, you could even tamper with the sun to have it emit a lot more radiation.
For any civilisation that can travel between the stars and has expanded from its home system, we are as weak as a small island nation against the Elizabethan British Empire.