Note that primary and secondary abilities give you different stats, regardless if they are stackable or not. For example, damage up primary gives you 10%, secondary gives you 3% (approximate values for example). And as previously mentioned, teach.hey will have diminishing effects as you stack more. The first Damage Up Primary applied gives a 9% damage boost, but 2 give ~8.25% each, and 3 give ~7.45%

I ve compared Seige of Shanghai and Hainan Resort, I wasn't able to find a difference in the actual maps (pc vs console)

Ya, I mainly play conquest large so I posted for those that I am aware of. Updated, thanks