Compared to just about everything else, Conquistador plain sucks. Poor fighter, comes far too late, prohibitive costs, no upgrade path whatsoever...yes, you can pillage, the problem is that your stupid conquistadores will get owned the very next turn, and the will just rebuild the road...not to mention, if for any reason the AI puts a musket over the resource, you can forget about the pillage with those weaklings...
The Flandes Corps (tercio) is historically a better UU, it was composed of different types of troops fighting in a single unit, but making that in civ is a little hard. It could just be 4/4/1 @ 70 shields replacing muskets? maybe a little overpowered, but they would require iron AND saltpeter