The cons of multiple human races:
1. Dwarf players will get several caravans in summer. They kill FPS and don't bring many unique useful goods. Even worse, they may be at war. By carefully (not too much) design you can avoid human and elven caravans to meet. But 5 human races, what the hell?
2. You prefer human to be the choice of starter players. But They will confuse at so many start choices. And they will keep trying different humans before they go to dwarves/orcs/cobolds. It will be long enough to lose patience and put MDF away.
3. Before you have designed goblin and elf, maybe also drow and antmen, it's hard to design multiple human races. It will be either too different, that some humans are more like dwarves and some are more like elves thus no human feel, or too same, that the differences are only renaming some weapons, skills and castes, thus not different feelings.
4. It does not fit in the game's world. While a race can found many civs, it's hard to say humans have divided into many RACES, before worldgen. Their difference should not equal that of elf and drow. We don't have humans living in the tree or living underground, and reproductive isolated over many generations.