I think good games are based on hard player choices. The hard choices are what makes a game interesting, it's up to you to decide which way to take. If there are no choices, then the game gets IMHO less interesting and challenging.
With this said, upgradeable units could be good if it was a difficult decision to make. If it was so expensive that 50% of players opted not to upgrade and the other half did - that would be good. But upgradeable units that players always would upgrade is IMHO not good.
So I don't want options that take away the difficulties of your choices. If you can back out from a choice, it's not a real choice. If you always will do an upgrade, it's not a choice.
Its already expensive, the faction I play costs 238 gold if I wanna upgrade leather to light plate armor for 4 men in a single unit.
I usually bite the bullet if such a unit is level 5 and up unless its an reserve army then it can make do with leather for longer. Then for that final Golem armor,, i believe its about 500g total if you wanna upgrade from leather to there while only having 4 men.
Obviously its a nobrainer to upgrade the guys in your main army first. Everything can't be always hard decision.
And upgrading from spears to boar spears and to pikes is another no brainer decision. It must be done if you want to have ease in combat instead of plinking away at enemy units for 1 damage.