UUs can still be a little spiky even if their unique promotions carry forward. They sometimes arrive at a tech earlier than the normal unit or have higher CS than the normal unit.
Right now the implementation doesn't seem consistent with any one overarching philosophy. If the philosophy should be "UUs should be spiky" then we'd probably want ALL UU promotions to be lost on upgrade. I'd favor an overarching philosophy of "unique promotions are fun so don't have them lost on upgrade if at all possible".
I probably would have proposed that all UU unique promotions remain on upgrade if I had the time to figure out a complete list of ones that are lost and then figure out appropriate ways to ensure they remain balanced. Pilum was just top of mind having recently played Rome.
Right now the implementation doesn't seem consistent with any one overarching philosophy. If the philosophy should be "UUs should be spiky" then we'd probably want ALL UU promotions to be lost on upgrade. I'd favor an overarching philosophy of "unique promotions are fun so don't have them lost on upgrade if at all possible".
I probably would have proposed that all UU unique promotions remain on upgrade if I had the time to figure out a complete list of ones that are lost and then figure out appropriate ways to ensure they remain balanced. Pilum was just top of mind having recently played Rome.