Some units don't have a promotion path to the higher tier despite blatantly having a higher tier counterpart, such as the militia - veteran militia and armed trader - caravan guard - veteran guard. Is there a reason that they are unable to be promoted? This is actually a rhetorical question, they should be upgradeable.
Well militia spawns are fixed, so that sort of make sense. You only get significant # of veteran militia in Towns/Castles that are doing well. Though it would be nice if Towns/Castles that were undisturbed effectively had better militia from them upgrading too. Militia units are all pretty bad, veterans are okay, but not particularly good for T4. I get rid of them soon as I can, only keep them if my party happens to be very diminished.
Caravan guards aren't terribly great either from what I recall, but suppose they can be decent. Why those don't upgrade is a mystery. Maybe they'd make caravans upkeep too expensive?
You can only build one caravan really anyways, the more caravans you build the faster they get destroyed. There's some funny TW math going on, because if you only have one caravan there's a good chance that it'll just run forever.
I hate caravans since they are rarely profitable as a result, but with shops being unreliable and extra expensive it's about only way to get passive income early on. Still don't think either are worth it now, unless you really want to RP a trader. Just kill people and take their stuff.