Specific units questions

[caravel]

A unit that is useful in desperation is still not a good unit. Peasants can save your skin if used correctly and at the right time. I would prefer a unit of Vanilla Spearmen to UM any day of the week. At least the latter can hold off cavalry for a little while. Militia Sergeants are much more worthwhile and I tend to skip UM and only train if absolutely necessary. UM are only dangerous if they're well upgraded and are under a good general, by that time you might as well train a better unit to replace the UM.

The only factions that find UM even remotely useful in early would be the HRE, Italians or French. The English have Fyrdmen (and Clansmen, Kerns and Gallowglasses!), The Danes - Vikings, the Spanish and Aragonese - Javelinmen, Jinetes, etc and the eastern factions have either Slav Javelins, Slav Warriors, Jobbagy, Vikings etc. The Turks and Byzantine don't need them - for obvious reasons, the Almohads certainly don't need them nor the Egyptians.

Of those factions that just might need UM, namely the French, HRE and Italians, I would still rather train Vanilla Spearmen. Also as the French have Hobilars to use as flankers early on that really leaves the UM somewhat redundant as the AP advantage is small considering that there won't be that many armoured units to deal with right at the start of the early era. This really leaves only the HRE and Italians with a possible need for UM at the beginning of the early era.