In the 15th century cavalry has an amazing advantage over infantry, but this edge is eroded as land tech increases. Check your ledger - one of the pages in it will list a shock and fire multiplier for your units - this number multiplies the values units get, so a 1-shock cavalry is actually 2 shock in 1453, while a 1-shock infantry is actually 0.5 shock. Infantry slowly gains both shock and fire, while cavalry essentially never gets much better than it starts with.
In addition to that, cavalry is faster and more manueverable - cavalry units can be used to outflank a smaller enemy force, so you will end up doing more damage without the cavalry getting hurt. Say you have 10 regiments and the enemy has only 6 - their six regiments will attack six of your regiments, and vice versa, but in addition, the four regiments on the ends can attack if they're manueverable enough - either a general or cavalry. They attack but can't be attacked back - so in this case (10 vs 6) the attacker will do almost twice as much damage per round!
Cavalry's big disadvantages are cost (to raise and maintain a cavalry regiment) and terrain. Cavalry suffers twice the usual terrain penalty, so they're not very effective in mountains or forests. However, only the attacker suffers terrain penalties. Since cavalry alone can outrun infantry, they are likely to reach a province first and become the defender. Besides this, even a -2 penalty may not outweigh the overall effectiveness of a cavalry regiment in battle in the early game.
Long story short: yes, you're probably not achieving much by tailoring your armies to the terrain, especially if you are the attacker/invader.
