TBH, the system they have (as with most things in the game) is workable, but the balance is presently off.
As I understand it, currently defense is calculated by taking the planetary population, multiplying it by Resistance, and then dividing it by the number of tiles on the planet. The problem with that is, it's a double-nerf on population - If I have a 20-tile planet with 100 population and 25% resistance, I end up with about 1.25 defenders per tile.
A 6-pop transport landing on this planet will lose defense % of his troops, and then the remaining ones will all be deployed against 1 tile at a time. Even invading a planet 75% defense, a single transport will land 1.5 troopers. Given attacker casualties rates are always lower than defender casualties, that single transport will likely be able to take 5-6 tiles before it is wiped out using conventional warfare. With any of the tactics which decrease resistance further, or anything that increases soldiering (which acts as a multiplier to attacker numbers), this becomes an automatic wipeout for the defender.
So here's what I think:
1) Defense is too powerful. Defense stat should pretty much never go over 50%, and getting it that high should require a lot of effort (like wonders and player-unique buildings). By combining an Elerium Defense Shield with an upgraded defense shield, you can put Defense to 125%, making a planet effectively immune to conventional warfare, planetary bombardment, and info war tactics, since they cannot reduce the defense to under 100%. This should Never Happen.
2) Resistance is too weak. Much too weak. It is basically operating as the counterpart to soldiering, but where soldiering starts at 100% resistance starts at 25%. This can be completely nulified by some of the invasion tactics. Info War, Planetary Bombardment, Tidal Disruption and Core Detonation can all reduce Resistance to <0%, which should also Never Happen.
3) Invasion tactics should not be on specialization techs. This may be controversial, but the fact is, if you make some unlucky choices you can be put in a position where you cannot invade some planets on a non-tech-trade game. If I throw 100 transports at a planet, that planet should fall regardless of if I use a stupid frontal assault or bombard the planet to bits; the real difference should be in my own casualty count. Specialization techs should instead offer choices between resistance-boosting effects and soldiering-boosting ones.
If we can get to a situation where Defense is never higher that 50% and Resistance is never lower than about that (but can go over 100%), then we might start seeing a more interesting invasion game with genuine arms races on the surface as well as in space.