I would say that barbs are not a factor, the AI will wipe that out at high levels as soon as they appear and I lower levels you should not have them in your land by that point in the game.
If you have barbarians on your hands in the late medieval period, you have a problem.
In general, I agree with both these comments, but I recently decided to go for a late conquest victory, instead of domination, so I was razing cities while going for the last of the Persians, and started seeing warriors come up, ride my railroads, and attack my tanks.

My first reaction was to put infantry on the hilltops to get rid of the fog, but then I realize that I could promote my tanks to elite status on the barbs before sending them into their first true battle. Got a whole bunch of tank armies out of that.
The only downside of the blitz ability is that if you put the units in armies, they actually lose some opportunities to attack. For instance, if you have 4 tanks, they could conceivably attack 8 times, if they were moved into position the turn before and you use a stack of artillery to redline the defenders. Those same 4 tanks in an army will only be able to attack four times, at a maximum. The army, however, can travel further in one turn, and can frequently take a city without artillery back-up, so there's definitely still a place for them, but I like to have some of both, for maximum flexibility.
As for Sipahi vs. Cossacks, Sipahi, but only by a little bit.