It seems like a very cool card but I have never once used it.
It is much better than the French crossbows to skirmishers card in the respect it changes the counter system. I imagine it would be an awesome late game card if you spammed axe riders, got them to build a heavy infantry counter (if make dragoons, don't bother), then send the card, tear up their army, and start scalping villagers. That’s the only realistic use.
If you look at it mathematically, and assume you send this card in late Age 3 when cards are worth approximately 1000 in resources (lets say 1000 wood as a worst case example which is worth 2000 villager seconds), each axe rider (160F 40G) is worth 257 vs, and each rifle rider (120F 100G) is worth 310 vs (not comparing any upgrades - just base 0.84 hunting and 0.6 mining), that is a savings of 52 vs per axe rider converted.
Therefore, on a worst case villager second comparison, you would need at least 38 axe riders to break even with a 1000 wood Fortress age shipment. This of course does not include the fact with what is more useful to have at the moment.
In other words, if you need it to counter something immediately, or just want a few rifle riders in Colonial (where there is no artillery, little cavalry, and few heavy infantry), it is a great card. Otherwise, go with something else. There are tons of great Age 2 cards for the Sioux.