btw, what the point of having to use Melee units at all if using any range matchlock rifles mount with horses especially with lances.

Answering the first question, possibly, the ashigaru gunner line is unique for Sakai culture, that is probably why they outperform the teppo ashigaru which anyone can get. So for effectiveness reason there really is no reason to use them, but I often end up using them when recruiting from prisoners or recruiting whatever is available in towns during tough wars.
As for the cavalry, would love to know it myself.
And as for why use melee units at all - to not get wiped out by enemy cavalry. Unless you can somehow train so many gunner cav consistently to always have numbers available. But they take so long to train, cost two different horse types to upgrade, that by time they reach good levels, some get shot by cheap enemy gunners. So again - logistics.

Edit: Guns of pistol and gun cavalry are not the same, just checked. Gunner one has 100 more damage and more missile speed, which at really long ranges means heavily armored enemies will die vs not dying.