Well, technically Eldar portals are the safest to use. The only more reliable long distance transportation method would be the Necron who are way out of everyones league and don't even use anything related to the warp.<br/><br/>The planet sounds like an excellent place to be ambushed by Lictors, and a 'huge underground city' as you suggested might very well be a<em>lot</em>worse and this could be a tomb world.