Planets are the core of an empire, and are probably the most significant strategic asset in Stellaris. Invading them should be a challenging and dynamic undertaking, not an afterthought to the orbital battle.
TLDR: This mod makes ground assaults more involved and longer lasting. Effects of orbital bombardment are now stronger, and more differentiated.
This mod is intended to make planetary invasions into a suitably impressive undertaking. In vanilla, they are rather bland and altogether too short. There is little strategic difference in types of bombardment, and once the defenses are reduced your armies can easily crush the defenders. With this mod, planetary fortifications provide less of a bonus, but are stronger and will only cause defenders to take normal (not 2x!) damage when they reach zero.
Army health has been increased relative to morale, so that it might require multiple assaults before a foothold on the planet’s surace is secure, or at the very least multiple waves of troops to be dropped. Generals now buff unit morale rather than health based on their skill level.
Damaged units now require 150% additional maintenance while being repaired, to represent the costly process of retraining and replacing casualties.
Armies and army attachments have been completed updated. There are now 3 quality tiers of assault and defense armies, as well as multiple types of clone units. Training times are significantly longer for more elite units, but their increased morale and combat stats makes them invaluable in difficult invasions.
Attachments now represent the general equipment that an army is equipped with. They will usually increase the maintenance cost of a unit while increasing performance. Power armor is a thing, as are personal shields and clone suicide brigades (a lot of these were coded in the language files but never implemented, interestingly enough).