They wouldn't be too much of a problem for the Europeans in an alt-19th century confrontation. European armies wiped the floor with horse nomads by then. But if you had horses and developed animal husbandry among the woodlands nations and in mesoamerica, things could well turn out rather differently early on.
Ah but remember the only got wiped outaround the late 19th century by semi-modern weaponry. 16th-18th are entierrly different