Honestly, the existence of shields alone would probably justify melee weapons. One of the key advantages of a bullet is that it doesn't need to penetrate through all of your armor, just the little bit where it hits. A shield generator basically allows you to concentrate 100% of your defensive capability into that same spot, so logically, it would take a dramatically bigger gun to penetrate that, possibly impractically big, because you are essentially talking about having to overcome the entire energy generation capacity of the reactor, which is Fusion or something along those lines. In practice, probably the most efficient way to take down an energy Shield would be to drive right up next to them, and hook your own reactor into the energy Shield via some sort of melee weapon. Otherwise, you are going to have all the inefficiencies of trying to convey energy efficiently over long distances, giving you an inherent disadvantage.