Well customization is a very wide subject.
We have physical customization where you can model and paint your force how YOU want to.
Everyone takes this as a given do they not?
IF we are talking about customizing how your force performs in game there are 2 factors.
Game play options, determined by the level of tactical complexity.
Model unit load out options , for strategic complication.
Games with more focus on tactical play , tend to have less complication in the strategic options.
Eg If generic light infantry can be used as skirmishers, scouts, raiders flanking force, distraction unit , forward observers for artillery etc.
Because the basic unit can be used in so many ways in game the unit load out customization can be less.
In games with very little tactical diversity, unit load out totally determines in game use.
So the level of game play customization tends to be around a sweet spot, but divided between tactical complexity and strategic complication.