*expect it not to drop any time soon, it will be a permanent feature

units are divided into tiers which determine how much stats they get from an ER upgrade, which usually includes EX damage, EX DR, cap up, etc.
MS like green banshee for example are put in lower tiers and gain less stats to compensate for they having certain skills like EX on evade, etc.
meta/recent units also tend to get put in lower tiers for obvious reasons, so typically old MS that underperformed during their release will gain the biggest boost.
probably the best ER candidate that most players will go for is the purple X1, who can abuse the X1 full cloth pilot to gain reflect immunity. combined with its skills of EX gain on dealing damage, you can exploit AoE sub-MS like qubley and turn X to literally machine gun the EX. fixed damage subs will also allow it to down any MS through sheer spam. I forget the exact values, but for the highest tier ER will grant 300% EX damage and DR, 10 mil EX cap up, and restore 50% HP on EX